[Cleanup] Use explicit conversions for enum formatting (#4064)

This is prep for updating to fmt 10 which removed implicit conversions
for enums.

common/patches/rof2_limits.h

@@ -158,6 +158,7 @@ namespace RoF2
 				slotCursor
 			};
 
+			constexpr int16 format_as(InventorySlots slot) { return static_cast<int16>(slot); }
 		} // namespace enum_
 		using namespace enum_;