libpmc: fix linking with C programs

Revision r334749 Added some C++ code to libpmc. It didn't change the ABI,
but it did introduce a dependency on libc++. Nobody noticed because every
program that in the base system that uses libpmc is also C++.

Reported-by: Dom Dwyer <dom@itsallbroken.com>
Reviewed By: vangyzen
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D28550