Fix gitattributes mismatch in meeting.ics

The build-source step was failing on the CI:

dpkg-source: info: building golang-github-opencontainers-specs using existing ./golang-github-opencontainers-specs_1.0.1.orig.tar.gz
dpkg-source: info: using patch list from debian/patches/series
dpkg-source: info: local changes detected, the modified files are:
 golang-github-opencontainers-specs-1.0.1/meeting.ics
dpkg-source: error: aborting due to unexpected upstream changes, see /tmp/golang-github-opencontainers-specs_1.0.1-5co1~5+2+g4b55322.diff.EWSl6G

This is due to the facts that:

1. this repository ships a .gitattributes file which sets text eol=crlf for all .ics files
2. the CI pipeline forces on all files the attributes -text -eol -crlf -ident -filter -working-tree-encoding -export-subst
3. the package was likely originally imported in the repository without the above override, so the meeting.ics file was committed in its "normalized" form with LF line terminators instead of the CRLF that are actually expected

Fix the situation by:

1. set the gitattributes override locally
2. extract meeting.ics from the orig tarball
3. commit it here in its "denormalized" form, with CRLF line endings