Fix 'make coverage' when used with lcov version 2.0+

Fix 'make coverage' when used with lcov version 2.0+

Starting with version 2.0, various lcov warnings now trigger an error
exit.  This results in 'make coverage' terminating before completion.

Fix this by invoking lcov and genhtml with --ignore-errors to prevent
the error exit when version 2.0+ is in use.

Manually tested by running elfutils-htdocs/update-coverage.sh with
lcov 1.14 and 2.0.

Signed-off-by: Aaron Merey <amerey@redhat.com>