C-family : Fix a C++ ODR violation [PR97854].

The changes in r11-4799 introduced a dummy enum rid type with
a different initial member name to the actual version (an ODR
violation).  Fixed by including the header declaring the
actual type.

gcc/c-family/ChangeLog:

	PR objc/97854
	* stub-objc.c: Include c-common.h to declare enum rid.

gcc/c-family/stub-objc.c

@@ -25,10 +25,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "tree.h"
 #include "vec.h"
 
-/* Provide a dummy type for the RID enum used as an argument to
-   objc_prop_attr_kind_for_rid () */
-enum rid { DUMMY };
-
+#include "c-common.h" /* for enum rid.  */
 #include "c-objc.h"
 
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