MIPS: kernel: setup.c: fix compilation error

With ath79_defconfig enabling CONFIG_MIPS_ELF_APPENDED_DTB gives a
compilation error. This patch fixes it.

Build log:
...
  CC      kernel/locking/percpu-rwsem.o
../arch/mips/kernel/setup.c:46:39: error: conflicting types for
'__appended_dtb'
 const char __section(".appended_dtb") __appended_dtb[0x100000];
                                       ^~~~~~~~~~~~~~
In file included from ../arch/mips/kernel/setup.c:34:
../arch/mips/include/asm/bootinfo.h:118:13: note: previous declaration
of '__appended_dtb' was here
 extern char __appended_dtb[];
             ^~~~~~~~~~~~~~
  CC      fs/attr.o
make[4]: *** [../scripts/Makefile.build:271: arch/mips/kernel/setup.o]
 Error 1
...

Root cause seems to be:
Fixes: b83ba0b9 ("MIPS: of: Introduce helper function to get DTB")

Signed-off-by: Mauri Sandberg <sandberg@mailfence.com>
Reviewed-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: trivial@kernel.org
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>