Version bump x11-libs/gtk+-2.24.21-r1

Portage version of gtk+:2 is up to 2.24.21 so I've merged the multilib
changes from 2.24.20-r1 to it and bumped it up to 2.24.21-r1.
Merge pull request #4 from mrpdaemon/gtk+2-bump

Version bump x11-libs/gtk+-2.24.21-r1