girepository: Use expected signed types for iterating

We are using various indexes types, but not always using the correct
sign or size, so let's adapt this to ensure we're consistent with the
values we're comparing with.

girepository/girffi.c

@@ -175,8 +175,7 @@ gi_callable_info_get_ffi_arg_types (GICallableInfo *callable_info,
 {
     ffi_type **arg_types;
     gboolean is_method, throws;
-    size_t n_invoke_args;
-    unsigned int n_args, i, offset;
+    size_t n_args, n_invoke_args, i, offset;
 
     g_return_val_if_fail (callable_info != NULL, NULL);