avoid name mangling

Otherwise the dynamic linker cannot find this function.
See also https://ci.appveyor.com/project/ruby/ruby/builds/27224231/job/4pg6lxlsnsjotu2l

ext/-test-/cxxanyargs/cxxanyargs.cpp

@@ -335,7 +335,7 @@ namespace test_rb_ivar_foreach {
     }
 }
 
-void
+extern "C" void
 Init_cxxanyargs(void)
 {
     VALUE b = rb_define_module("Bug");