LinuxKPI: add fips.h

We do not currently aim to support crypto FIPS compliance in LinuxKPI
so keep this turned off.

Approved by: re (cperciva)
Reviewed by: emaste, dumbbell
Differential Revision: https://reviews.freebsd.org/D53207

(cherry picked from commit 98609f4fecffd0e70293ddd57c2b785e498b1131)
(cherry picked from commit 3aaaba20786966062d1355d4809142b623ffc4b7)