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

drivers/avr/ssd1306.h

@@ -1,5 +1,4 @@
-#ifndef SSD1306_H
-#define SSD1306_H
+#pragma once
 
 #include <stdbool.h>
 #include <stdio.h>
@@ -86,5 +85,3 @@ void matrix_write_char(struct CharacterMatrix *matrix, uint8_t c);
 void matrix_write(struct CharacterMatrix *matrix, const char *data);
 void matrix_write_P(struct CharacterMatrix *matrix, const char *data);
 void matrix_render(struct CharacterMatrix *matrix);
-
-#endif