C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/guiVolumeChange.h

@@ -17,8 +17,7 @@ ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */
 
-#ifndef GUIVOLUMECHANGE_HEADER
-#define GUIVOLUMECHANGE_HEADER
+#pragma once
 
 #include "irrlichttypes_extrabloated.h"
 #include "modalMenu.h"
@@ -44,6 +43,3 @@ public:
 
 	bool pausesGame() { return true; }
 };
-
-#endif
-