src/borg/cache_sync/unpack.h: fix compiler warnings

The key argument being sent to hashindex_get and hashindex_set by
multiple functions is a different signedness from what the functions
expect. This resolves the issue by changing the key type in the
unpack_user struct to unsigned char.

src/borg/cache_sync/unpack.h

@@ -102,7 +102,7 @@ typedef struct unpack_user {
 
     /* collect values here for current chunklist entry */
     struct {
-        char key[32];
+        unsigned char key[32];
         uint32_t csize;
         uint32_t size;
     } current;