selftests/clone3: Fix build error

When compiling the selftests with the -std=gnu99 option the build can
fail with.

Following build error:

  test_core.c: In function ‘test_cgcore_destroy’:
  test_core.c:87:2: error: ‘for’ loop initial declarations are only
  allowed in C99 mode
    for (int i = 0; i < 10; i++) {
    ^
  test_core.c:87:2: note: use option -std=c99 or -std=gnu99 to compile

Add -std=gnu99 to the clone3 selftest Makefile to fix this.

Signed-off-by: Xingxing Su <suxingxing@loongson.cn>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>