Add average cell voltage to OSD

This can make it a little easier for some pilots to see where their
battery is at, especially if they move forth and back between different
cell counts a lot.

src/main/io/osd.h

@@ -48,6 +48,7 @@ typedef enum {
     OSD_POWER,
     OSD_PIDRATE_PROFILE,
     OSD_MAIN_BATT_WARNING,
+    OSD_AVG_CELL_VOLTAGE,
     OSD_ITEM_COUNT // MUST BE LAST
 } osd_items_e;