[CMAKE] Determine the Qt library directory from a public variable

Summary:
The cmake script currently uses a private variable to get the Qt library
directory. This diffs determines this location from the Qt5Core library
location, avoiding the use of the private variable.
It should have no impact on the build.

Test Plan:

cmake -GNinja ..
ninja
./src/qt/bitcoin-qt

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2947