Makefile.am: move gtk-doc.make back to EXTRA_DIST

0192c599 moved this file from EXTRA_DIST TO
BUILT_EXTRA_DIST for an unknown reason.

Having it here causes the timestamp to be updated during 'make dist' to
something newer than docs/reference/*/Makefile.in.  This is a problem
because those Makefile.in are generated by including gtk-doc.make, so
automake becomes convinced that they need to be regenerated.  This is a
problem for people who don't have automake installed, or have the wrong
version.

The timestamp problem has been around for quite a while, but it only
became a problem recently after automake changed policy about this in
commit a22717dffe37f30ef2ad2c355b68c9b3b5e4b8c7.

Specifically:

https://bugzilla.gnome.org/show_bug.cgi?id=700350