Change include guards in quantum/ to pragma once (#11239)

quantum/process_keycode/process_printer.h

@@ -14,13 +14,10 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#ifndef PROCESS_PRINTER_H
-#define PROCESS_PRINTER_H
+#pragma once
 
 #include "quantum.h"
 
 #include "protocol/serial.h"
 
 bool process_printer(uint16_t keycode, keyrecord_t *record);
-
-#endif