Add 'extern int optopt;' for Linux and perhaps others.

Reported by Remi Collet.
diff --git a/src/ziptool.c b/src/ziptool.c
index d319bfb..062267e 100644
--- a/src/ziptool.c
+++ b/src/ziptool.c
@@ -48,6 +48,7 @@
 #ifndef HAVE_GETOPT
 #include "getopt.h"
 #endif
+extern int optopt;
 
 #include "zip.h"
 #include "compat.h"