lcov: filter /usr/lib/ from coverage reports

Summary: Backport of core PR14281.

Test Plan:

../configure --enable-lcov
make
make cov

Check the total.coverage directory does not contain any usr/ subdir
anymore.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5301