C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/guiFormSpecMenu.h

@@ -17,9 +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 GUIINVENTORYMENU_HEADER
-#define GUIINVENTORYMENU_HEADER
+#pragma once
 
 #include <utility>
 #include <stack>
@@ -567,5 +565,3 @@ public:
 
 	std::string m_formspec;
 };
-
-#endif