Eliminate 'unused parameter' gcc warning in free() if IGNORE_FREE

(a cherry-pick of commit c3e77d2 from 'master')

PR #598 (bdwgc).

* malloc.c [REDIRECT_FREE && !REDIRECT_MALLOC_IN_HEADER]
(free): Specify p is an unused argument.

malloc.c

@@ -675,7 +675,7 @@ GC_API void GC_CALL GC_free(void * p)
 #   define REDIRECT_FREE_F REDIRECT_FREE
 # endif
 
-  void free(void * p)
+  void free(void * p GC_ATTR_UNUSED)
   {
 #   ifndef IGNORE_FREE
 #     if defined(GC_LINUX_THREADS) && !defined(USE_PROC_FOR_LIBRARIES)