if /usr/ports is a git repo checkout, dune will look into
/usr/ports/.git for a version string.

fix this by adding a version string to dune-project.

Index: dune-project
--- dune-project.orig
+++ dune-project
@@ -1,2 +1,3 @@
 (lang dune 1.0)
 (name yojson)
+(version 1.7.0)
