BUILD: activity/memprofile: fix a build warning in the posix_memalign handler

A "return NULL" statement was placed for error handling in the
posix_memalign() handler instead of an int errno value, by recent
commit 5ddc8b3ad4 ("MINOR: activity/memprofile: monitor non-portable
calls as well"). Surprisingly the warning only triggered on gcc-4.8.
Let's use ENOMEM instead. No backport needed.

src/activity.c

@@ -282,7 +282,7 @@ static int memprof_posix_memalign_initial_handler(void **ptr, size_t al, size_t
 {
 	if (in_memprof) {
 		/* probably that dlsym() needs posix_memalign(), let's fail */
-		return NULL;
+		return ENOMEM;
 	}
 
 	memprof_init();