hwpmc: add error for HWPMC_DEBUG without KTR

The option is a no-op otherwise, but this is not necessarily obvious.
Failing the compile gives the user a hint.

Reviewed by: jkoshy, emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40544

(cherry picked from commit 569f89b2f442e2187f750a5892a1662390364cc6)