Add missing instance unlock

When using the apteryx mainloop from lua directly (not via Alfred) the
instance must be unlocked to hand control of the state over to the
library to use.

lua.c

@@ -1102,6 +1102,7 @@ lua_apteryx_mainloop (lua_State *L)
     }
 
     running = true;
+    lua_apteryx_instance_unlock (L);
     while (running && fd >= 0)
     {
         g_L = L;