C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/database-postgresql.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 DATABASE_POSTGRESQL_HEADER
-#define DATABASE_POSTGRESQL_HEADER
+#pragma once
 
 #include <string>
 #include <libpq-fe.h>
@@ -145,6 +144,3 @@ protected:
 private:
 	bool playerDataExists(const std::string &playername);
 };
-
-#endif
-