build: Uninstall JavaScript and CSS from htmldir

Otherwise, distcheck fails when mallard-ducktype is available.
Signed-off-by: Simon McVittie <smcv@collabora.com>

doc/Makefile.am

@@ -132,6 +132,8 @@ uninstall-local::
 	rm -f $(DESTDIR)$(apidir)/*.png
 	rm -f $(DESTDIR)$(apidir)/*.css
 	rm -f $(DESTDIR)$(apidir)/*.js
+	rm -f $(DESTDIR)$(htmldir)/*.css
+	rm -f $(DESTDIR)$(htmldir)/*.js
 	rm -f $(DESTDIR)$(htmldir)/*.html
 	rm -f $(DESTDIR)$(docdir)/*.txt
 	rm -f $(DESTDIR)$(htmldir)/*.png