Use <poll.h> instead of <sys/poll.h>

The former should be used according to POSIX, otherwise it causes
bunches of warnings when compiling for musl-based distros like Alpine
Linux.

src/dnsmasq.h

@@ -95,7 +95,11 @@ typedef unsigned long long u64;
 #if defined(HAVE_SOLARIS_NETWORK)
 #  include <sys/sockio.h>
 #endif
-#include <sys/poll.h>
+#if defined(HAVE_POLL_H)
+#  include <poll.h>
+#else
+#  include <sys/poll.h>
+#endif
 #include <sys/wait.h>
 #include <sys/time.h>
 #include <sys/un.h>