When building malta_defconfig with CONFIG_RELOCATABLE=y, there are two
warnings due to missing prototypes for functions only used when that
configuration is enabled:

  arch/mips/kernel/relocate.c:42:12: warning: no previous prototype for 'plat_post_relocation' [-Wmissing-prototypes]
     42 | int __weak plat_post_relocation(long offset)
        |            ^~~~~~~~~~~~~~~~~~~~
  arch/mips/kernel/relocate.c:324:14: warning: no previous prototype for 'relocate_kernel' [-Wmissing-prototypes]
    324 | void *__init relocate_kernel(void)
        |              ^~~~~~~~~~~~~~~

While relocate_kernel() is only called from assembly, it makes sense to
keep the prototypes together in C to fix the warnings. Add them to
silence the warnings.
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>