Allocator: fix build error with -Werror=format-security

Doing it again here, because I fixed this only in master branch before..

intern/guardedalloc/intern/mallocn_lockfree_impl.c

@@ -102,7 +102,7 @@ size_t MEM_lockfree_allocN_len(const void *vmemh)
 void MEM_lockfree_freeN(void *vmemh)
 {
   if (leak_detector_has_run) {
-    print_error(free_after_leak_detection_message);
+    print_error("%s\n", free_after_leak_detection_message);
   }
 
   MemHead *memh = MEMHEAD_FROM_PTR(vmemh);