vndk-def: Fix vndk-core path assertion.

This commit fixes an assertion in the vndk-core computation.  The
assertion should check the path of the newly added generic library.
Before this commit, it is checking the input library, which might be
outward-customized.

Test: ./tests/run.py
Change-Id: I15bd7c122a4442281bc735e16b6c615ea8190411