selftests/clone3: Fix compiler warning

Shuah reported a compiler warning with an Ubuntu GCC 13 build, I've been
unable to reproduce it but hopefully this fixes the issue:

clone3_set_tid.c:136:43: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘size_t’ {aka ‘long unsigned int’} [-Wformat=]
Reported-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/clone3/clone3_set_tid.c

@@ -133,7 +133,7 @@ static void test_clone3_set_tid(const char *desc,
 		"[%d] clone3() with CLONE_SET_TID %d says: %d - expected %d\n",
 		getpid(), set_tid[0], ret, expected);
 
-	ksft_test_result(ret == expected, "%s with %d TIDs and flags 0x%x\n",
+	ksft_test_result(ret == expected, "%s with %zu TIDs and flags 0x%x\n",
 			 desc, set_tid_size, flags);
 }