lantiq: lzma-loader: fix mismatched function declaration

lantiq: lzma-loader: fix mismatched function declaration

This fix compilation warning:
printf.c:14:13: warning: type of 'board_putc' does not match original declaration [-Wlto-type-mismatch]
   14 | extern void board_putc(int ch);
      |             ^
board-lantiq.c:24:6: note: type mismatch in parameter 1
   24 | void board_putc(char c)
      |      ^
board-lantiq.c:24:6: note: type 'char' should match type 'int'
board-lantiq.c:24:6: note: 'board_putc' was previously declared here

Fixes: a328b6831c0f ("lantiq: bring back okli loader")
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>