win32ole: separate global variable declarations and definitions

https://gcc.gnu.org/gcc-10/changes.html#c

> * GCC now defaults to `-fno-common`.  As a result, global
>   variable accesses are more efficient on various targets.  In
>   C, global variables with multiple tentative definitions now
>   result in linker errors.  With `-fcommon` such definitions are
>   silently merged during linking.

ext/win32ole/win32ole_method.c

@@ -923,6 +923,8 @@ folemethod_inspect(VALUE self)
     return default_inspect(self, "WIN32OLE_METHOD");
 }
 
+VALUE cWIN32OLE_METHOD;
+
 void Init_win32ole_method(void)
 {
     cWIN32OLE_METHOD = rb_define_class("WIN32OLE_METHOD", rb_cObject);