lcov: filter /usr/lib/ from coverage reports
lcov: filter /usr/lib/ from coverage reports

Summary: Backport of core [[https://github.com/bitcoin/bitcoin/pull/14281 | 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

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