Clean gnome-doc-utils.make in target "dist-clean"

According to common autotools rules this should be cleaned in maintainer mode only.

Signed-off-by: Rob Norris <rw_norris@hotmail.com>

Makefile.am

@@ -25,8 +25,10 @@ generate-changelog:
 dist-hook: viking.spec generate-changelog
 	cp $(top_builddir)/viking.spec $(distdir)
 
+MAINTAINERCLEANFILES = \
+	gnome-doc-utils.make
+
 DISTCLEANFILES = \
-	gnome-doc-utils.make \
 	intltool-extract \
 	intltool-merge \
 	intltool-update