journal: use poll.h rather than sys/poll.h

This does not change anything, as poll.h is a one-line wrapper of sys/poll.h.
Note that man pages e.g. poll(2) indicate to include poll.h rather than sys/poll.h.
So, let's use poll.h.

src/journal/journald-sync.c

@@ -1,8 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include <linux/sockios.h>
+#include <poll.h>
 #include <sys/ioctl.h>
-#include <sys/poll.h>
 
 #include "sd-varlink.h"