GUI2: use include guards consistently

[ci skip]

Doesn't touch uses of `#pragma once` outside the gui/ directory.

src/gui/dialogs/drop_down_menu.hpp

@@ -12,7 +12,8 @@
    See the COPYING file for more details.
 */
 
-#pragma once
+#ifndef GUI_DIALOGS_DROP_DOWN_MENU_HPP_INCLUDED
+#define GUI_DIALOGS_DROP_DOWN_MENU_HPP_INCLUDED
 
 #include "gui/dialogs/modal_dialog.hpp"
 #include "sdl/rect.hpp"
@@ -92,3 +93,5 @@ private:
 
 } // namespace dialogs
 } // namespace gui2
+
+#endif

src/gui/dialogs/loading_screen.hpp

@@ -11,7 +11,8 @@
    See the COPYING file for more details.
 */
 
-#pragma once
+#ifndef GUI_DIALOGS_LOADING_SCREEN_HPP_INCLUDED
+#define GUI_DIALOGS_LOADING_SCREEN_HPP_INCLUDED
 
 #include "gui/dialogs/modal_dialog.hpp"
 #include "tstring.hpp"
@@ -100,3 +101,5 @@ private:
 
 } // namespace dialogs
 } // namespace gui2
+
+#endif

src/gui/widgets/chatbox.hpp

@@ -11,7 +11,8 @@
    See the COPYING file for more details.
 */
 
-#pragma once
+#ifndef GUI_WIDGETS_CHATBOX_HPP_INCLUDED
+#define GUI_WIDGETS_CHATBOX_HPP_INCLUDED
 
 #include "gui/widgets/container_base.hpp"
 #include "game_initialization/lobby_data.hpp"
@@ -290,3 +291,5 @@ private:
 // }------------ END --------------
 
 } // namespace gui2
+
+#endif

src/gui/widgets/menu_button.hpp

@@ -12,7 +12,8 @@
    See the COPYING file for more details.
 */
 
-#pragma once
+#ifndef GUI_WIDGETS_MENU_BUTTON_HPP_INCLUDED
+#define GUI_WIDGETS_MENU_BUTTON_HPP_INCLUDED
 
 #include "gui/core/widget_definition.hpp"
 #include "gui/core/window_builder.hpp"
@@ -197,3 +198,5 @@ private:
 // }------------ END --------------
 
 } // namespace gui2
+
+#endif

src/gui/widgets/multimenu_button.hpp

@@ -12,7 +12,8 @@
    See the COPYING file for more details.
 */
 
-#pragma once
+#ifndef GUI_WIDGETS_MULTIMENU_BUTTON_HPP_INCLUDED
+#define GUI_WIDGETS_MULTIMENU_BUTTON_HPP_INCLUDED
 
 #include "gui/core/widget_definition.hpp"
 #include "gui/core/window_builder.hpp"
@@ -250,3 +251,5 @@ private:
 // }------------ END --------------
 
 } // namespace gui2
+
+#endif