selftests: pidfd: fix compilation errors due to wait.h

Drop unneeded <linux/wait.h> header inclusion to fix pidfd compilation
errors seen in Fedora 32:

In file included from pidfd_open_test.c:9:
../../../../usr/include/linux/wait.h:17:16: error: expected identifier before numeric constant
   17 | #define P_ALL  0
      |                ^

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

Author: rantala

tools/testing/selftests/pidfd/pidfd_open_test.c

@@ -6,7 +6,6 @@
 #include <inttypes.h>
 #include <limits.h>
 #include <linux/types.h>
-#include <linux/wait.h>
 #include <sched.h>
 #include <signal.h>
 #include <stdbool.h>

tools/testing/selftests/pidfd/pidfd_poll_test.c

@@ -3,7 +3,6 @@
 #define _GNU_SOURCE
 #include <errno.h>
 #include <linux/types.h>
-#include <linux/wait.h>
 #include <poll.h>
 #include <signal.h>
 #include <stdbool.h>