netlink/netlink.h: As the poll() man page recommends include the <poll.h>
instead of <sys/poll.h>. This removes an error when building libdl against 
the musl C library.

Signed-off-by: Jörg Krause <jkrause@posteo.de>

diff -purN libnl-3.2.25.orig/include/netlink/netlink.h libnl-3.2.25/include/netlink/netlink.h
--- libnl-3.2.25.orig/include/netlink/netlink.h	2014-09-01 11:42:37.254724124 +0200
+++ libnl-3.2.25/include/netlink/netlink.h	2014-09-01 11:42:58.254724255 +0200
@@ -16,7 +16,7 @@
 #include <stdint.h>
 #include <string.h>
 #include <stdlib.h>
-#include <sys/poll.h>
+#include <poll.h>
 #include <sys/socket.h>
 #include <sys/types.h>
 #include <sys/time.h>