C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/objdef.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 OBJDEF_HEADER
-#define OBJDEF_HEADER
+#pragma once
 
 #include "util/basic_macros.h"
 #include "porting.h"
@@ -93,5 +92,3 @@ protected:
 	std::vector<ObjDef *> m_objects;
 	ObjDefType m_objtype;
 };
-
-#endif