vmspawn: 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.

Similar to 8139906eaa.
Follow-up for 0fc45c8d20.

src/vmspawn/vmspawn.c

@@ -1,10 +1,10 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include <getopt.h>
+#include <poll.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <sys/poll.h>
 #include <sys/stat.h>
 #include <unistd.h>