Make source seek/tell functions public.
diff --git a/lib/zip_source_tell.c b/lib/zip_source_tell.c
index cbbfdfa..f1c10b5 100644
--- a/lib/zip_source_tell.c
+++ b/lib/zip_source_tell.c
@@ -35,7 +35,7 @@
 #include "zipint.h"
 
 
-zip_int64_t
+ZIP_EXTERN zip_int64_t
 zip_source_tell(zip_source_t *src)
 {
     if (src->source_closed) {