d2r2: fix missing function types on functions returning void

src/linux/window_manager/linux_window_manager.h

@@ -1,8 +1,8 @@
 // Copyright (c) Epic Games Tools
 // Licensed under the MIT license (https://opensource.org/license/mit/)
 
-#ifndef OS_GFX_LINUX_H
-#define OS_GFX_LINUX_H
+#ifndef LINUX_WINDOW_MANAGER_H
+#define LINUX_WINDOW_MANAGER_H
 
 ////////////////////////////////
 //~ rjf: Includes
@@ -58,4 +58,4 @@ global LNX_WM_GfxState *os_lnx_gfx_state = 0;
 
 internal LNX_WM_Window *os_lnx_window_from_x11window(Window window);
 
-#endif // OS_GFX_LINUX_H
+#endif // LINUX_WINDOW_MANAGER_H