diff-highlight: add clean target to Makefile

Now that `make` produces a file, we should have a clean target to remove
it.

Signed-off-by: Daniel Watkins <daniel@daniel-watkins.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/diff-highlight/Makefile

@@ -17,4 +17,7 @@ shebang.perl: FORCE
 test: all
 	$(MAKE) -C t
 
+clean:
+	$(RM) diff-highlight
+
 .PHONY: FORCE