Fix coverage with lcov 2.0 and uncovered subprojects

Version 2.0 of lcov triggers an error when an exclude pattern is unused.  This
can happen when the project has subprojects, but no code in them ends up
covered:

lcov: ERROR: 'exclude' pattern '/[...]/subprojects/*' is unused.
	(use "lcov --ignore-errors unused ..." to bypass this error)

So, simply do as it says.  Unused patterns doesn't seem problematic (or even
interesting) here, so I don't think there's any risk in simply turning this
off, which matches what happened with earlier versions of lcov anyway.

mesonbuild/scripts/coverage.py

@@ -145,6 +145,7 @@ def coverage(outputs: T.List[str], source_root: str, subproject_root: str, build
                                    '--remove', covinfo,
                                    *lcov_subpoject_exclude,
                                    *lcov_exe_rc_branch_coverage,
+                                   '--ignore-errors', 'unused',
                                    '--output-file', covinfo] + lcov_config)
             subprocess.check_call([genhtml_exe,
                                    '--prefix', build_root,