Include linux/processor.h to fix build error:

arch/mips/sgi-ip30/ip30-console.c: In function ‘prom_putchar’:
arch/mips/sgi-ip30/ip30-console.c:21:17: error: implicit declaration of function ‘cpu_relax’ [-Werror=implicit-function-declaration]
   21 |                 cpu_relax();

Cc: stable@vger.kernel.org
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>