extensions: add Make rules for *-gtk-doc.h
extensions: add Make rules for *-gtk-doc.h

3c830e9 fixed `make clean` but in the process it broke building from
nothing… since these are now included in
nodist_libgabble_extensions_la_SOURCES, we need to tell Make how to
build them.

Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble