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

tmk_core/common/virtser.h

@@ -1,10 +1,7 @@
-#ifndef _VIRTSER_H_
-#define _VIRTSER_H_
+#pragma once
 
 /* Define this function in your code to process incoming bytes */
 void virtser_recv(const uint8_t ch);
 
 /* Call this to send a character over the Virtual Serial Device */
 void virtser_send(const uint8_t byte);
-
-#endif