selftests: pidfd: drop needless linux/kcmp.h inclusion in pidfd_setns_test.c

kcmp is not used in pidfd_setns_test.c, so do not include <linux/kcmp.h>

Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>