m68k: sun3: Annotate prom_printf() with __printf()

When building with W=1:

    arch/m68k/sun3/prom/printf.c: In function ‘prom_printf’:
    arch/m68k/sun3/prom/printf.c:35:9: warning: function ‘prom_printf’ might be a candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
       35 |         vsprintf(ppbuf, fmt, args);
	  |         ^~~~~~~~

Fix this by annotating prom_printf() with __printf(1, 2).

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/61017c012617e6f633ac9f9a3c2659ae38e1f2fb.1694613528.git.geert@linux-m68k.org