powerpc/mm: change #include "mmu_decl.h" to <mm/mmu_decl.h>

This patch make inclusion of mmu_decl.h independant of the location
of the file including it.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>