net.h: don't use the reserved name __unused

The __* keywords are reserved. On FreeBSD __unused evaluates
to the attribute unused, causing a compilation failure.
Just use unused instead.
Signed-off-by: NJeroen Hofstee <jeroen@myspectrum.nl>
cc: joe.hershberger@gmail.com

include/net.h

@@ -357,7 +357,7 @@ struct icmp_hdr {
 		} echo;
 		ulong	gateway;
 		struct {
-			ushort	__unused;
+			ushort	unused;
 			ushort	mtu;
 		} frag;
 		uchar data[0];