glsl2: Don't declare a variable called sig that shadows the other one

Accidentally having a variable called 'sig' within an if-statement
cause the higher scope 'sig' to always be NULL.  As a result a new
function signature was created for a function definition even when one
already existed from a prototype declaration.

Fixes piglit test case glsl-function-prototype (bugzilla #29520).

src/glsl/ast_to_hir.cpp

@@ -2128,7 +2128,7 @@ ast_function::hir(exec_list *instructions,
     */
    f = state->symbols->get_function(name);
    if (f != NULL) {
-      ir_function_signature *sig = f->exact_matching_signature(&hir_parameters);
+      sig = f->exact_matching_signature(&hir_parameters);
       if (sig != NULL) {
 	 const char *badvar = sig->qualifiers_match(&hir_parameters);
 	 if (badvar != NULL) {