C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/guiTable.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 GUITABLE_HEADER
-#define GUITABLE_HEADER
+#pragma once
 
 #include <map>
 #include <set>
@@ -256,6 +254,3 @@ protected:
 	static void alignContent(Cell *cell, s32 xmax, s32 content_width,
 			s32 align);
 };
-
-#endif
-