xdiff: mark unused parameter in xdl_call_hunk_func()

This function is used interchangeably with xdl_emit via a function
pointer, so we can't just drop the unused parameter. Mark it to silence
-Wunused-parameter.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

xdiff/xdiffi.c

@@ -973,7 +973,7 @@ void xdl_free_script(xdchange_t *xscr) {
 	}
 }
 
-static int xdl_call_hunk_func(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
+static int xdl_call_hunk_func(xdfenv_t *xe UNUSED, xdchange_t *xscr, xdemitcb_t *ecb,
 			      xdemitconf_t const *xecfg)
 {
 	xdchange_t *xch, *xche;