compiler,glsl: fix warning when -finstrument-functions is used

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28411>

src/compiler/glsl/ast_to_hir.cpp

@@ -2707,6 +2707,8 @@ get_type_name_for_precision_qualifier(const glsl_type *type)
    default:
       unreachable("Unsupported type");
    } /* base type */
+
+   return NULL;
 }
 
 static unsigned