It wouldn't be a problem to change the debian package to include these headers, but pd itself doesn't install them.
right ... in theory no external developer should use these headers, but what does grep -r g_canvas.h /path/to/pd/externals tell you? in fact, you can't build a lot of externals without these headers ... but on the other hand, no one has to use cyclone, flext, iemlib, toxy, yves externals or your gui externals ... ;-)
Then, there is a debian package for flext, so why not use this and solve the compilation problem together (againandagainandagainandagain)
well, there are people out there, who would like to make use of the features in devel like having a threadsafe gui or lockfree thread synchronisation using idle callbacks ...
cheers ... tim