diff: remove unused #include "sigchain.h"

After switching to use the tempfile module in commit 284098f1
(diff: use tempfile module), no declarations from sigchain.h are used in
diff.c anymore. Thus, remove the #include.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -13,7 +13,6 @@
 #include "run-command.h"
 #include "utf8.h"
 #include "userdiff.h"
-#include "sigchain.h"
 #include "submodule.h"
 #include "ll-merge.h"
 #include "string-list.h"