fs/Makefile: Stupid typo breakage of exofs inclusion

In my last patch I did a stupid mistake and broke the exofs
compilation completely. Fix it ASAP.

Instead of obj-y I did obj-$(y)

Really Really sorry. Me totally blushing :-{|

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>