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_variant.c

@@ -689,6 +689,8 @@ ole_variant2variant(VALUE val, VARIANT *var)
     VariantCopy(var, &(pvar->var));
 }
 
+VALUE cWIN32OLE_VARIANT;
+
 void
 Init_win32ole_variant(void)
 {