add missing header to have malloc/free available

lib/zip_source_file_win32.h

diff --git a/lib/zip_source_file_win32.h b/lib/zip_source_file_win32.h
index 23172a1..88e45b1 100644
--- a/lib/zip_source_file_win32.h
+++ b/lib/zip_source_file_win32.h
@@ -43,6 +43,8 @@
 
 #include <aclapi.h>
 
+#include <stdlib.h>
+
 #include "zipint.h"
 
 #include "zip_source_file.h"