build: conditional include adg-canvas.h

adg-canvas.h is created at configure time from a template. On in-place
builds (i.e. where $srcdir is .), the *.h glob of docs/adg/Makefile.am
properly includes that file. On VPATH builds instead, that file must be
manually included in EXTRA_HFILE.

It is required to include any file once (and only once) otherwise
gtk-doc will start barking and bad things will happen.