C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/inventory.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 INVENTORY_HEADER
-#define INVENTORY_HEADER
+#pragma once
 
 #include "debug.h"
 #include "itemdef.h"
@@ -310,5 +309,3 @@ private:
 	IItemDefManager *m_itemdef;
 	bool m_dirty = false;
 };
-
-#endif