glsl: Suppress warning from matching_signature change.

gcc isn't smart enough to see that we only look at matched_score after
we've initialized it (because match != NULL happens at the same time)

src/glsl/ir_function.cpp

@@ -165,7 +165,7 @@ ir_function_signature *
 ir_function::matching_signature(const exec_list *actual_parameters)
 {
    ir_function_signature *match = NULL;
-   int matched_score;
+   int matched_score = 0;
 
    foreach_iter(exec_list_iterator, iter, signatures) {
       ir_function_signature *const sig =