test-sizeof: also add uid_t

We had gid_t, and pid_t, but not uid_t. Add for completeness.

src/test/test-sizeof.c

@@ -62,6 +62,7 @@ int main(void) {
         info(usec_t);
         info(__time_t);
         info(pid_t);
+        info(uid_t);
         info(gid_t);
 
         info(enum Enum);