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

rust-lang/libc,rust-lang/libc,rust-lang/libc