ooo-file-unreg.c: Include poll.h instead of sys/poll.h

This fixes a warning e.g.

In file included from ooo-file-unreg.c:12:
/mnt/b/yoe/master/build/tmp/work/riscv32-yoe-linux-musl/liburing/2.7/recipe-sysroot/usr/include/sys/poll.h:1:2: warning: redirecting incorrect #include <sys/poll.h> to <poll.h> [-W#warnings]
    1 | #warning redirecting incorrect #include <sys/poll.h> to <poll.h>
      |  ^
1 warning generated.

Signed-off-by: Khem Raj <raj.khem@gmail.com>