selftests: pidfd: Fix undefined reference to pthread_create()

Fix build failure:

undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status

Fix CFLAGS to include pthread correctly.

Fixes: 740378dc ("pidfd: add polling selftests")
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20190924195237.30519-1-skhan@linuxfoundation.org


Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>