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/20241213-work-mount-rbtree-lockless-v3-9-6e3cdaf9b280@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

tools/testing/selftests/pidfd/pidfd.h

@@ -12,7 +12,6 @@
 #include <stdlib.h>
 #include <string.h>
 #include <syscall.h>
-#include <sys/mount.h>
 #include <sys/types.h>
 #include <sys/wait.h>