community/njs: upgrade to 0.7.12

ref https://nginx.org/en/docs/njs/changes.html#njs0.7.12

@jirutka There's new xml module added but I did not add libxml2 dependency as it works without it --no-libxml2 but tests don't.

I gonna build unit with njs support but as I see no make libnjs happening and no libraries packaged. Should I build it using source or we can package libraries and headers? Unit using 0.7.10 as default, see https://github.com/nginx/unit/commit/f4298180ebae2de1658684b0dc6604387b372a73