Fix build with -fno-common

Index: fsdiff.c
--- fsdiff.c.orig
+++ fsdiff.c
@@ -28,6 +28,7 @@ extern char	*version, *checksumlist;
 
 void		fs_walk( char *, struct stat *, char *, struct applefileinfo *,
 	int, int, int );
+int		cksum;
 int		dodots = 0;
 int		dotfd;
 int		lastpercent = -1;