Change from sys/poll.h to standard poll.h location

According to "man 2 poll" the correct header to include is poll.h, not
sys/poll.h. Reported by a build against musl.

Change-Id: I5298b49dc947d1a368e423104c0c0c7b9bdd1a10
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-on: http://openocd.zylin.com/2947
Tested-by: jenkins

src/helper/system.h

@@ -56,8 +56,8 @@
 #ifdef HAVE_SYS_SOCKET_H
 #include <sys/socket.h>
 #endif
-#ifdef HAVE_SYS_POLL_H
-#include <sys/poll.h>
+#ifdef HAVE_POLL_H
+#include <poll.h>
 #endif
 
 #ifdef __ECOS