debian/rules: Use --buildsystem=cmake+ninja

This doesn't seem to make much of a difference in build time. I read that cmake+ninja is faster than cmake if you are doing an incremental build though.

qtwebkit-opensource-src just switched to cmake+ninja.

I've been thinking about making this change for a while but I didn't do it earlier because the required debhelper version wasn't part of stretch, but that's less important now (and it's easy to revert this change for a stretch branch if we want).