Include stdarg.h unconditionally in StringUtil.cpp

[meyerj]

This might fix a compile error with GCC 5.5:

/home/travis/build/orocos-toolchain/orocos_toolchain/log4cpp/src/StringUtil.cpp: In static member function ‘static std::__cxx11::string log4cpp::StringUtil::vform(const char*, __va_list_tag*)’:
/home/travis/build/orocos-toolchain/orocos_toolchain/log4cpp/src/StringUtil.cpp:47:36: error: ‘va_copy’ was not declared in this scope
             va_copy(args_copy, args);
                                    ^

Edit: This error also occurs with the default GCC version 5.4 in Ubuntu Xenial when building without C++11 support enabled.

[meyerj]

The patch does not fix the problem. For some reason LOG4CPP_HAVE_SNPRINTF is not defined.
Might be related to #14 because I am building with ccache.

[meyerj]

va_copy was only added to C++ in C++11.
If you compile without -std=c++03 or with -std=gnu++03, va_copy is defined.