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

tmk_core/common/sendchar.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 SENDCHAR_H
-#define SENDCHAR_H
+#pragma once
 
 #include <stdint.h>
 
@@ -30,5 +29,3 @@ int8_t sendchar(uint8_t c);
 #ifdef __cplusplus
 }
 #endif
-
-#endif