Blender Git Loki

Fix linker errors when building without Cycles

Fix linker errors by explicitly linking PugiXML when OpenImageIO is
used.

PugiXML has been separated from OpenImageIO (D8628). This means that any
time OpenImageIO libraries are linked, the PugiXML libraries have to be
linked as well. This was set up correctly for Cycles, but not for imbuf.
Because of this, building without Cycles but with OIIO would produce
linker errors.