Use correct <poll.h> include

* `<sys/poll.h>` is not specified in POSIX

src/shared/nscd-flush.c

@@ -1,5 +1,5 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
-#include <sys/poll.h>
+#include <poll.h>
 
 #include "fd-util.h"
 #include "io-util.h"

src/shared/varlink.c

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include <malloc.h>
-#include <sys/poll.h>
+#include <poll.h>
 
 #include "alloc-util.h"
 #include "errno-util.h"