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

tmk_core/common/uart.h

@@ -1,5 +1,4 @@
-#ifndef _uart_included_h_
-#define _uart_included_h_
+#pragma once
 
 #include <stdint.h>
 
@@ -7,5 +6,3 @@ void    uart_init(uint32_t baud);
 void    uart_putchar(uint8_t c);
 uint8_t uart_getchar(void);
 uint8_t uart_available(void);
-
-#endif