[JavaRuntime] Add module.modulemap

Fix a build error in 6.1 toolchain. Apparently some targets have both
`JavaRuntime-tool.build` and `JavaRuntime.build` as the include paths.
The auto generated `module.modulemap` in both directories cause the
error `error: redefinition of module 'JavaRuntime'`. To avoid that,
add `module.modulemap` in the source directory so it's not generated.

Fixes: #229

Author: rintaro

Sources/JavaRuntime/include/module.modulemap

@@ -0,0 +1,4 @@
+module JavaRuntime {
+    umbrella header "JavaRuntime.h"
+    export *
+}