powerpc/8xx: add missing header in 8xx_mmu.c

arch/powerpc/mm/8xx_mmu.c:174:6: error: no previous prototype for ‘set_context’ [-Werror=missing-prototypes]
 void set_context(unsigned long id, pgd_t *pgd)

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <oss@buserror.net>