Fix a compiler warning.

Author: arthurschreiber

ext/rugged/rugged_refdb_backend_custom.c

@@ -93,7 +93,7 @@ static VALUE rb_git_refdb_backend_custom_new(VALUE self, VALUE rb_repo)
 	backend->parent.exists = &rugged_refdb_backend_custom__exists;
 	backend->parent.compress = &rugged_refdb_backend_custom__compress;
 	backend->parent.lookup = &rugged_refdb_backend_custom__lookup;
-	backend->parent.free = xfree;
+	backend->parent.free = (void (*)(git_refdb_backend *)) &xfree;
 
 	backend->self = Data_Wrap_Struct(self, NULL, rb_git_refdb_backend__free, backend);
 	return backend->self;