[POWERPC] fix implicit declaration on cell.

(Only fails with -Werror-implicit-function-declaration, but
 it should still be fixed).

arch/powerpc/platforms/cell/setup.c:145: error: implicit declaration of function 'udbg_init_rtas_console'

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>