pan.cc: fix one gdk_pixbuf_new_from_inline warning
This function is deprecated.
Current situation:
- pixbuf are loaded from a data structure declared in pan-pixbufs.h
- pixbufs.h is generated at build time from the pnf files stored in pan/icons
With this patch:
- pixbuf is constructed from png file
- pan-pixbufs.h is no longer needed, at least for this icon (there are a gazillion others to take care of).
- icons now need to be shipped with pan.
- this is handled by cmake build, but not by autotools.