Change include guards in tmk_core/ and drivers/ to pragma once (#11240)

tmk_core/common/timer.h

@@ -15,8 +15,7 @@ You should have received a copy of the GNU General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#ifndef TIMER_H
-#define TIMER_H 1
+#pragma once
 
 #include <stdint.h>
 #include <stdbool.h>
@@ -51,5 +50,3 @@ uint32_t timer_elapsed32(uint32_t last);
 #ifdef __cplusplus
 }
 #endif
-
-#endif