pv-runtime: Correct sense of a comparison

If the library is found in a directory not matching /*/lib/MULTIARCH, we would have wrongly guessed that ${exec_prefix} is the root directory by deleting a path that didn't match the known patterns. Of course what was intended here is to delete a path that does match the known patterns.

Fixes: 485269b6 "pv-runtime: Use _srt_get_path_after to remove provider prefix"