ir_function: Make matching_signature not return const

The linker needs to use this function to get specific function signatures, but
it also needs to modify the returned signature.  Since this method isn't itself
const (i.e., const this pointer), there is no value in making a const and
non-const version.

src/glsl/ir_function.cpp

@@ -155,7 +155,7 @@ parameter_lists_match(exec_list *list_a, exec_list *list_b)
 }
 
 
-const ir_function_signature *
+ir_function_signature *
 ir_function::matching_signature(exec_list *actual_parameters)
 {
    ir_function_signature *match = NULL;