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

quantum/api.h

@@ -14,8 +14,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#ifndef _API_H_
-#define _API_H_
+#pragma once
 
 #ifdef __AVR__
 #    include "lufa.h"
@@ -54,5 +53,3 @@ __attribute__((weak)) bool process_api_quantum(uint8_t length, uint8_t* data);
 __attribute__((weak)) bool process_api_keyboard(uint8_t length, uint8_t* data);
 
 __attribute__((weak)) bool process_api_user(uint8_t length, uint8_t* data);
-
-#endif