Fix build error on Windows with HEAD

[Watson1978]

When I built the master branch code on Windows,
I got the following error.

PS C:\src\calyptia-cmetrics-ruby> ridk exec rake
mkdir -p tmp/x64-mingw-ucrt/cmetrics/3.4.1
cd tmp/x64-mingw-ucrt/cmetrics/3.4.1
C:/Ruby34-x64/bin/ruby.exe -I. ../../../../ext/cmetrics/extconf.rb
checking for whether cmake3 or cmake is usable... cmake
Downloading v0.5.9 (100%)

... (snip) ...

[ 88%] Building C object src/CMakeFiles/cmetrics-static.dir/cmt_decode_msgpack.c.obj
bash.exe: warning: could not find /tmp, please create!
C:/src/calyptia-cmetrics-ruby/tmp/x64-mingw-ucrt/cmetrics/3.3.6/tmp/x86_64-w64-mingw32/ports/cmetrics/0.5.9/cmetrics-0.5.9/src/cmt_decode_msgpack.c: In function 'unpack_basic_type_meta':
C:/src/calyptia-cmetrics-ruby/tmp/x64-mingw-ucrt/cmetrics/3.3.6/tmp/x86_64-w64-mingw32/ports/cmetrics/0.5.9/cmetrics-0.5.9/src/cmt_decode_msgpack.c:998:21: error: assignment to 'struct cmt_counter *' from incompatible pointer type 'struct counter *' [-Wincompatible-pointer-types]
  998 |             counter = (struct counter *) decode_context->map->parent;
      |                     ^
make[2]: *** [src/CMakeFiles/cmetrics-static.dir/build.make:410: src/CMakeFiles/cmetrics-static.dir/cmt_decode_msgpack.c.obj] Error 1
make[1]: *** [CMakeFiles/Makefile2:435: src/CMakeFiles/cmetrics-static.dir/all] Error 2
make: *** [Makefile:156: all] Error 2
----- end of file -----
*** ../../../../ext/cmetrics/extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include=${opt-dir}/include
        --without-opt-include
        --with-opt-lib=${opt-dir}/lib
        --without-opt-lib
        --with-make-prog
        --without-make-prog
        --srcdir=../../../../ext/cmetrics
        --curdir
        --ruby=C:/Ruby34-x64/bin/$(RUBY_BASE_NAME)
C:/Ruby34-x64/lib/ruby/gems/3.4.0/gems/mini_portile2-2.8.8/lib/mini_portile2/mini_portile.rb:627:in 'block in MiniPortile#execute': Failed to complete compile task (RuntimeError)
        from C:/Ruby34-x64/lib/ruby/gems/3.4.0/gems/mini_portile2-2.8.8/lib/mini_portile2/mini_portile.rb:593:in 'Dir.chdir'
        from C:/Ruby34-x64/lib/ruby/gems/3.4.0/gems/mini_portile2-2.8.8/lib/mini_portile2/mini_portile.rb:593:in 'MiniPortile#execute'
        from C:/Ruby34-x64/lib/ruby/gems/3.4.0/gems/mini_portile2-2.8.8/lib/mini_portile2/mini_portile.rb:194:in 'MiniPortile#compile'
        from ../../../../ext/cmetrics/extconf.rb:121:in 'BuildCMetrics#build'
        from ../../../../ext/cmetrics/extconf.rb:145:in '<main>'
rake aborted!
Command failed with status (1): [C:/Ruby34-x64/bin/ruby.exe -I. ../../../../ext/cmetrics/extconf.rb]

Tasks: TOP => default => compile => compile:x64-mingw-ucrt => compile:cmetrics:x64-mingw-ucrt => copy:cmetrics:x64-mingw-ucrt:3.4.1 => tmp/x64-mingw-ucrt/cmetrics/3.4.1/cmetrics.so => tmp/x64-mingw-ucrt/cmetrics/3.4.1/Makefile
(See full trace by running task with --trace)

This patch will avoid above compile error on Windows.