tests/sys/pmc: only build if MK_PMC != no

This unbreaks the build when pmc support is explicitly disabled via the
aforementioned build knob.

MFC after: 10 days
Fixes: 2cfd82f74 ("hwpmc: add regression tests for ...")
Differential Revision: https://reviews.freebsd.org/D58401

(cherry picked from commit a18e773d2776a65c1a2a0418cbec9cf5ef526b53)