add proper dependancies on the xdiff source
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: Andy Whitcroft <b4658fdd7fbd645c2859b0e39d71018f32058d12@shadowen.org>
Acked-by: Ryan Anderson <ea3cd978650417470535f3a4725b6b5042a6ab59@google.com>
Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git