Declare struct ucred in handlers.h and netlink.h

Both files refer to it.  The sources including those files are not
guaranteed to include sys/socket.h to ensure that struct ucred is
defined.

include/netlink/handlers.h

@@ -25,6 +25,7 @@ extern "C" {
 struct nl_cb;
 struct nl_sock;
 struct nl_msg;
+struct ucred;
 
 /**
  * @name Callback Typedefs

include/netlink/netlink.h

@@ -36,6 +36,8 @@
 extern "C" {
 #endif
 
+struct ucred;
+
 extern int nl_debug;
 extern struct nl_dump_params nl_debug_dp;