cbfstool: fix implicit declaration of strcasecmp

Change-Id: Iefeb47bca3676a1f807b7a66b74a07491e351362
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/27632
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

util/cbfstool/cbfscomptool.c

@@ -16,6 +16,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <strings.h>
 #include <time.h>
 
 #include "common.h"