Submitted upstream at https://github.com/Swordfish90/cool-retro-term/issues/692

--- a/qmltermwidget/lib/TerminalCharacterDecoder.cpp	1969-12-31 18:00:01.000000000 -0600
+++ b/qmltermwidget/lib/TerminalCharacterDecoder.cpp	2021-12-27 22:07:04.452256118 -0600
@@ -19,6 +19,8 @@
     02110-1301  USA.
 */
 
+#include <cwctype>
+
 // Own
 #include "TerminalCharacterDecoder.h"