icon-theme: Fix multiple inherits in a single theme
icon-theme: Fix multiple inherits in a single theme

I forgot to replace commas with NULLs in the Inherits string, which broke
the inherits of any theme that had more than one inherited theme.

pwithnall/mx,jonnylamb/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx