Makefile: clean up http-walker.o dependency rules

http-walker.o depends on http.h twice: once in the rule listing
files that use http.h, and again in the rule explaining how to
build it.  Messy.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

Makefile

@@ -1628,7 +1628,7 @@ http.o: http.c GIT-CFLAGS
 	$(QUIET_CC)$(CC) -o $*.o -c $(ALL_CFLAGS) -DGIT_USER_AGENT='"git/$(GIT_VERSION)"' $<
 
 ifdef NO_EXPAT
-http-walker.o: http-walker.c http.h GIT-CFLAGS
+http-walker.o: http-walker.c GIT-CFLAGS
 	$(QUIET_CC)$(CC) -o $*.o -c $(ALL_CFLAGS) -DNO_EXPAT $<
 endif