Always use <poll.h>

Previously we were always using <sys/poll.h> since
HAVE_POLL_H is never set. This looks like an autoconfism
that has crept in, but we don't use autoconf.

poll.h is the correct header file, as far as I can tell.

src/dnsmasq.h

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