twmsg: fix build error (isdigit not declared)

Change-Id: I3101d15e5d94241cf8e583d1e06e09f3bc8b31f7
diff --git a/gui/twmsg.cpp b/gui/twmsg.cpp
index 00c6931..f9ffefc 100644
--- a/gui/twmsg.cpp
+++ b/gui/twmsg.cpp
@@ -21,6 +21,7 @@
 #include "resources.hpp"
 
 #include "twmsg.h"
+#include <cctype>
 
 std::string Message::GetFormatString(const std::string& name) const
 {