powerpc/e500: Add missing prototype for 'relocate_init'

Kernel test robot reports:

 arch/powerpc/mm/nohash/e500.c:314:21: warning: no previous prototype for 'relocate_init' [-Wmissing-prototypes]
     314 | notrace void __init relocate_init(u64 dt_ptr, phys_addr_t start)
         |                     ^~~~~~~~~~~~~

Add it in mm/mmu_decl.h, close to associated is_second_reloc
variable declaration.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/oe-kbuild-all/202302181136.wgyCKUcs-lkp@intel.com/
Link: https://lore.kernel.org/r/ac9107acf24135e1a07e8f84d2090572d43e3fe4.1676712510.git.christophe.leroy@csgroup.eu