Fix poll.h include path

The correct include path for poll(2) is poll.h not sys/poll.h.
sys/poll.h may not be available on some libcs or may issue a
warning. In particular this fixes a warning on musl systems.

src/st-util/gdb-remote.c

@@ -12,7 +12,7 @@
 #include <win32_socket.h>
 #else
 #include <unistd.h>
-#include <sys/poll.h>
+#include <poll.h>
 #endif
 
 #include "gdb-remote.h"