rts: Add missing const in HEAP_ALLOCED_GC

This was previously unnoticed as this code-path is hit on very few
platforms (e.g. OpenBSD).

rts/sm/HeapAlloc.h

@@ -192,7 +192,7 @@ StgBool HEAP_ALLOCED(const void *p)
 // updated atomically, but we need to place a lock around operations
 // that touch the MBlock map.
 INLINE_HEADER
-StgBool HEAP_ALLOCED_GC(void *p)
+StgBool HEAP_ALLOCED_GC(const void *p)
 {
     StgWord mblock;
     uint32_t entry_no;