selftests: remove unneeded include

The pidfd header will be included in a sample program and this pulls in
all the mount definitions that would be causing problems.

Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

tools/testing/selftests/pidfd/pidfd.h

···
 #include <stdlib.h>
 #include <string.h>
 #include <syscall.h>
-#include <sys/mount.h>
 #include <sys/types.h>
 #include <sys/wait.h>