Add missing ZIP_EXTERN to function definitions.

lib/zip_source_begin_write.c

diff --git a/lib/zip_source_begin_write.c b/lib/zip_source_begin_write.c
index b3675d2..0459337 100644
--- a/lib/zip_source_begin_write.c
+++ b/lib/zip_source_begin_write.c
@@ -35,7 +35,7 @@
 #include "zipint.h"
 
 
-int
+ZIP_EXTERN int
 zip_source_begin_write(zip_source_t *src)
 {
     if (ZIP_SOURCE_IS_OPEN_WRITING(src)) {