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 <rpjday@crashcourse.ca>
Cc: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>