Add setting for inflation dates mode

src/saveload/extended_ver_sl.h

@@ -98,6 +98,7 @@ enum SlXvFeatureIndex {
 	XSLFI_NEWGRF_INFO_EXTRA,                      ///< Extra NewGRF info in savegame
 	XSLFI_INDUSTRY_CARGO_ADJ,                     ///< Industry cargo adjustment patch
 	XSLFI_REALISTIC_TRAIN_BRAKING,                ///< Realistic train braking
+	XSLFI_INFLATION_FIXED_DATES,                  ///< Inflation is applied between fixed dates
 
 	XSLFI_RIFF_HEADER_60_BIT,                     ///< Size field in RIFF chunk header is 60 bit
 	XSLFI_HEIGHT_8_BIT,                           ///< Map tile height is 8 bit instead of 4 bit, but savegame version may be before this became true in trunk