libc-test: include linux/sched.h test_linux()

The UAPI version of sched.h contains the newest flags that may not have
been added to the libc's sched.h yet (for example CLONE_PIDFD). It was
alreaded included by test_android(), but was missing from test_linux().

Author: neocturne

libc-test/build.rs

@@ -2654,6 +2654,7 @@ fn test_linux(target: &str) {
         "linux/random.h",
         "linux/reboot.h",
         "linux/rtnetlink.h",
+        "linux/sched.h",
         "linux/seccomp.h",
         "linux/sockios.h",
         "linux/uinput.h",