Add ZIP_TRUNCATE, based on php's OVERWRITE.

--HG--
branch : HEAD
diff --git a/lib/zip.h b/lib/zip.h
index b34039f..b98a060 100644
--- a/lib/zip.h
+++ b/lib/zip.h
@@ -59,6 +59,7 @@
 #define ZIP_CREATE           1
 #define ZIP_EXCL             2
 #define ZIP_CHECKCONS        4
+#define ZIP_TRUNCATE         8
 
 
 /* flags for zip_name_locate, zip_fopen, zip_stat, ... */