Add -lpfm to pkg-config Libs.private when libpfm is enabled

[kiwigitops]

Summary

When BENCHMARK_ENABLE_LIBPFM=ON, libpfm is linked into the benchmark library via the PFM::libpfm imported target. The pkg-config generation derives Libs.private from the target's LINK_LIBRARIES, but the loop only emits -l flags for entries that are not CMake targets. Imported targets like PFM::libpfm are silently skipped, so -lpfm never makes it into benchmark.pc.

As a result, projects that statically link benchmark through pkg-config fail to resolve pfm_initialize / pfm_get_os_event_encoding at link time.

This adds -lpfm to the private link libraries for the PFM::libpfm target, alongside the existing private dependencies (-lpthread, etc.).

Closes #1908.

Testing

• git diff --check

[LebedevRI]

This should probably be using get_target_property() instead of
guessing the lib name, oh, and, this seems to completely not handle -L?

[kiwigitops]

Good points, updated. It now resolves the target via get_target_property(... IMPORTED_LOCATION) and derives the flags from the resolved path instead of hardcoding -lpfm, emitting both -L<dir> and -l<name>. So PFM::libpfm (or any imported private dependency) contributes its directory and library name to Libs.private.