patch_imm64_load_insns() is only used locally in
arch_prepare_optimized_kprobe() and does not need to be external.

It fixes this W=1 compile error :

../arch/powerpc/kernel/optprobes.c:149:6: error: no previous prototype for ‘patch_imm64_load_insns’ [-Werror=missing-prototypes]
  149 | void patch_imm64_load_insns(unsigned int val, kprobe_opcode_t *addr)
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-12-clg@kaod.org