C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/chatmessage.h

@@ -17,8 +17,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
 
-#ifndef MT_CHATMESSAGE_H
-#define MT_CHATMESSAGE_H
+#pragma once
 
 #include <string>
 #include <ctime>
@@ -47,5 +46,3 @@ struct ChatMessage
 	std::wstring sender = L"";
 	std::time_t timestamp = std::time(0);
 };
-
-#endif