common/crc32c_intel_fast: fix compile-time #ifdef

This wasn't getting built in!

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
(cherry picked from commit 3233336cc3b6c2c1e89fe6c6d21d42e0f2cce142)