BUILD: memprof: make the old caller pointer a const in get_prof_bin()

It's a const void* in the target, we can't use a void* in the caller,
this causes a build warning with clang.
diff --git a/src/activity.c b/src/activity.c
index 2080fd6..179ff1f 100644
--- a/src/activity.c
+++ b/src/activity.c
@@ -187,7 +187,7 @@
 static struct memprof_stats *memprof_get_bin(const void *ra)
 {
 	int retries = 16; // up to 16 consecutive entries may be tested.
-	void *old;
+	const void *old;
 	unsigned int bin;
 
 	bin = memprof_hash_ptr(ra);