Add fallback text, missing #include.

Author: TomHarte

src/tokeniser.cpp

@@ -3,6 +3,7 @@
 
 #include <cctype>
 #include <cstdio>
+#include <functional>
 #include <map>
 
 /*

src/tokeniser.hpp

@@ -25,6 +25,7 @@ struct Error {
 				case Type::LineTooLong:			return "LineTooLong";
 				case Type::BadStringLiteral:	return "BadStringLiteral";
 			}
+			return "InvalidError";
 		}() + " at line " + std::to_string(line_number);
 	}
 };