day2/vhdl: workaround ghdl#1529

https://github.com/ghdl/ghdl/pull/1529

2020/day2/vhdl/parser.vhd

@@ -30,7 +30,7 @@ architecture behaviour of parser is
 
 	function char_to_digit(input : in character) return digit is
 	begin
-		if input >= '0' and input <= '9' then
+		if not (input < '0') and input <= '9' then
 			return character'pos(input) - character'pos('0');
 		else
 			return 0;