Makefile: add missing GIT-VERSION-FILE dependency

In 20fc9bc5 (Set HTTP user agent to git/GIT_VERSION, 2006-04-04),
http.o started recording GIT_VERSION, but http.o wasn't added
to the list of files that depends on GIT-VERSION-FILE.

Fix this, so mofications to GIT-VERSION-FILE will result in an
updated user-agent string.
Signed-off-by: NErik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -1874,7 +1874,7 @@ configure: configure.ac
 	$(RM) $<+
 
 # These can record GIT_VERSION
-git.o git.spec \
+git.o git.spec http.o \
 	$(patsubst %.sh,%,$(SCRIPT_SH)) \
 	$(patsubst %.perl,%,$(SCRIPT_PERL)) \
 	: GIT-VERSION-FILE