libcaja-private: fix -Wlogical-not-parentheses build warning

fixes this warning:
caja-undostack-manager.c: In function 'retrieve_files_to_restore':
caja-undostack-manager.c:1967:38: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
   if (!(g_hash_table_size (trashed)) > 0) {
                                      ^
caja-undostack-manager.c:1967:7: note: add parentheses around left hand side expression to silence this warning
   if (!(g_hash_table_size (trashed)) > 0) {
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       (                             )

libcaja-private/caja-undostack-manager.c

@@ -1964,7 +1964,7 @@ get_uri_parent_path (char *uri)
 static GHashTable *
 retrieve_files_to_restore (GHashTable * trashed)
 {
-  if (!(g_hash_table_size (trashed)) > 0) {
+  if ((!(g_hash_table_size (trashed))) > 0) {
     return NULL;
   }