common: don't try to redefine PANIC if it already exists
common: don't try to redefine PANIC if it already exists

canvas_base.c tries to define PANIC, but it might already be set
if eg client/debug.h has been included before. All the other
macros in this file are guarded by #ifndef, this commit adds
the missing #ifndef to PANIC. Note that this is just a bandaid,
ideally common/ would contain a logging frameword, and these
macros would only be defined once instead of being defined in
several places.

SPICE/spice,freedesktop-unofficial-mirror/spice__spice,dezelin/spice,fgouget/spice,alon/spice-server,mathslinux/spice,dezelin/spice,alon/spice-server,Fantu/spice,freedesktop-unofficial-mirror/spice__spice,freedesktop-unofficial-mirror/spice__spice,Fantu/spice,ya-mouse/spice,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,ya-mouse/spice,alon/spice-server,alon/spice-server,mathslinux/spice,elmarco/spice,SPICE/spice,Fantu/spice,freedesktop-unofficial-mirror/spice__spice,dezelin/spice,alon/spice-server,elmarco/spice,elmarco/spice,elmarco/spice,mathslinux/spice,fgouget/spice,mathslinux/spice,dezelin/spice,mathslinux/spice,elmarco/spice,SPICE/spice,ya-mouse/spice,Fantu/spice,dezelin/spice,fgouget/spice