add proper dependancies on the xdiff source

We are not rebuilding the xdiff library when its header files change.
Add dependancies for those to the main Makefile.
Signed-off-by: NAndy Whitcroft <apw@shadowen.org>
Acked-by: NRyan Anderson <ryan@google.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Makefile

@@ -760,6 +760,8 @@ $(LIB_FILE): $(LIB_OBJS)
 	rm -f $@ && $(AR) rcs $@ $(LIB_OBJS)
 
 XDIFF_OBJS=xdiff/xdiffi.o xdiff/xprepare.o xdiff/xutils.o xdiff/xemit.o
+$(XDIFF_OBJS): xdiff/xinclude.h xdiff/xmacros.h xdiff/xdiff.h xdiff/xtypes.h \
+	xdiff/xutils.h xdiff/xprepare.h xdiff/xdiffi.h xdiff/xemit.h
 
 $(XDIFF_LIB): $(XDIFF_OBJS)
 	rm -f $@ && $(AR) rcs $@ $(XDIFF_OBJS)