sysv fs: remove superfluous check for __GNUC__ compiler

Since <linux/sysv_fs.h> isn't exported to userspace, there is little
point checking that this is a GNU-compatible compiler.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>