bug#27467: Xfce broken, because it propagates two different versions of gtk+

[Mark H Weaver]

I just tried to update my GuixSD system to commit
c57b56722f6c167c5a285f47802047de85a356ae on master.  "guix system build"
failed with the following error:

guix system: error: profile contains conflicting entries for gtk+:out
guix system: error:   first entry: gtk+@2.24.31:out 
/gnu/store/901ify6fr2zlr2y2acpvlqr8lygmwj98-gtk+-2.24.31
guix system: error:    ... propagated from exo@0.10.3
guix system: error:    ... propagated from xfce@4.12.0
guix system: error:   second entry: gtk+@3.22.15:out 
/gnu/store/fhqaljm8cf78irhqjbrm8n0ri1k2cznm-gtk+-3.22.15
guix system: error:    ... propagated from libxfce4ui@4.12.0
guix system: error:    ... propagated from garcon@0.4.0
guix system: error:    ... propagated from xfce@4.12.0

       Mark