mech: must specify model revision

Found when debugging output from "show state", which displayed NULL for
the infix model.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/mech/yang/infix.yang

@@ -28,4 +28,11 @@ module infix {
     description
         "Base model
          ";
+
+    revision 2023-02-10 {
+      description
+	"Initial revision.";
+      reference
+	"N/A";
+    }
 }