Comment # 26 on bug 1059438 from Joachim Wagner

@Carlos Looking at the patch of comment #21 and searching for a file "pc" in a
"symbols" folder I found /usr/share/X11/xkb/symbols/pc and it has those lines.
Try  putting ", Multi_key" back in on your system. Probably that's all.

(The patch is a bit difficult to read as on first glance it seems to add the
multikey but actually it's a patch inside a patch and the inner patch
previously added the multikey and with the new patch the inner patch is
removed.)