Makefile: add missing dependencies on url.h

v1.7.2-rc0~56^2 and its parent (decode file:// and ssh://
URLs, 2010-05-23) introduced a new url library.  Update the
Makefile with the relevant dependencies.

Detected with "make CHECK_HEADER_DEPENDENCIES=1".

Cc: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1854,6 +1854,7 @@ builtin/prune.o builtin/reflog.o reachable.o: reachable.h
 builtin/commit.o builtin/revert.o wt-status.o: wt-status.h
 builtin/tar-tree.o archive-tar.o: tar.h
 builtin/pack-objects.o: thread-utils.h
+connect.o transport.o http-backend.o: url.h
 http-fetch.o http-walker.o remote-curl.o transport.o walker.o: walker.h
 http.o http-walker.o http-push.o remote-curl.o: http.h