leak-detective: Register OS X specific hooks just once

If we initialize libstrongswan more than once in the same process, we may
not register the hooks twice.

src/libstrongswan/utils/leak_detective.c

@@ -333,9 +333,16 @@ HOOK(size_t, size, const void *ptr)
  */
 static bool register_hooks()
 {
+	static bool once = FALSE;
 	malloc_zone_t *zone;
 	void *page;
 
+	if (once)
+	{
+		return TRUE;
+	}
+	once = TRUE;
+
 	zone = malloc_default_zone();
 	if (zone->version != MALLOC_ZONE_VERSION)
 	{