Try to fix compile error on win32

https://github.com/ruby/ruby/runs/213995386#step:7:810
```
cxxanyargs.cpp
C:\Program Files (x86)\Windows Kits\10\include\10.0.17763.0\ucrt\corecrt_malloc.h(54): error C2485: '__restrict': unrecognized extended attribute
```

win32/Makefile.sub

@@ -861,11 +861,13 @@ $(CONFIG_H): $(MKFILES) $(srcdir)/win32/Makefile.sub $(win_srcdir)/Makefile.sub
 #define RUBY_LONGJMP(env,val) longjmp(env,val)
 #define RUBY_JMP_BUF jmp_buf
 #define inline __inline
+#ifndef __cplusplus
 !if $(MSC_VER) >= 1800
 #define restrict __restrict
 !else
 #define restrict /* not supported */
 !endif
+#endif
 #define NEED_IO_SEEK_BETWEEN_RW 1
 !if "$(MACHINE)" == "x86" || "$(ARCH)" == "x64"
 #define STACK_GROW_DIRECTION -1