OpenSSL: install .pc files from the exporters subdir

[ngie]

Summary

The .pc files generated in the root directory are used as part of the
build; they should never be installed. Use the versions from the
exporters subdirectory, which should be installed, as the .pc files
which are distributed with FreeBSD.

Reported by:
MFC after: 3 days

Test Plan

Confirmed that the version/copy of the file now properly installs from crypto/openssl . Prior to this change a version from the original import (3.5.1) was installed in all scenarios (which was incorrect).