vndk-def: Add non-AOSP lib to imported_ext_symbols.

This commit adds non-AOSP libraries to imported_ext_symbols even when no
symbols are resolved to that specific non-AOSP library.  This should fix
some errors related to the missing libraries from generic references.

Test: ./tests/run.py
Change-Id: I7f104737308d4dc0b591266d3408b001325c959e