powerpc/prom_init: Make "fake_elf" const

It is never modified
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/prom_init.c

@@ -922,7 +922,7 @@ struct ibm_arch_vec __cacheline_aligned ibm_architecture_vec = {
 
 /* Old method - ELF header with PT_NOTE sections only works on BE */
 #ifdef __BIG_ENDIAN__
-static struct fake_elf {
+static const struct fake_elf {
 	Elf32_Ehdr	elfhdr;
 	Elf32_Phdr	phdr[2];
 	struct chrpnote {