removes more dependencies

port/picolibc/port.c

@@ -1,8 +1,6 @@
 #include <stdio.h>
 #include <sys/types.h>
-
-extern void uart_putc(char c);
-extern int uart_getc(void);
+#include "board.h"
 
 static int
 board_putc(char c, FILE *file)