Otherwise the build fails because prom_init is calling symbols it's
not allowed to, eg:

  Error: External symbol 'ftrace_likely_update' referenced from prom_init.c
  make[3]: *** [arch/powerpc/kernel/Makefile:197: arch/powerpc/kernel/prom_init_check] Error 1
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191106051129.7626-1-mpe@ellerman.id.au