nbd: delete superfluous test for __GNUC__

Since <linux/compiler.h> already tests for __GNUC__, there's no point in nbd.h
repeating that test.

Signed-off-by: Robert P. J. Day <[e-mail]>
Cc: Paul Clements <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/nbd.h

···
 	char handle[8];
 	__be64 from;
 	__be32 len;
-}
-#ifdef __GNUC__
-	__attribute__ ((packed))
-#endif
-;
+} __attribute__ ((packed));
 
 /*
  * This is the reply packet that nbd-server sends back to the client after