META_MODE: Fix not writing .meta files in the kernel build.

This was a regression in r320220 due to improper porting of the
same logic from share/mk/bsd.dep.mk and having only tested with
-DNO_FILEMON at the time.

Pointyhat to: bdrewery
Reported by: Mark Millard, dhw, O. Hartmann
Sponsored by: Dell EMC Isilon