kernel: fix MIPS memory function optimization build error in decompressor

Fixes the following build error:
  CC      arch/mips/boot/compressed/decompress.o
  OBJCOPY arch/mips/boot/compressed/vmlinux.bin
  XZKERN  arch/mips/boot/compressed/vmlinux.bin.z
  OBJCOPY arch/mips/boot/compressed/piggy.o
  LD      vmlinuz
arch/mips/boot/compressed/decompress.o: In function `lzma2_lzma':
/tmp2/mehrtens/linux/arch/mips/boot/compressed/../../../../lib/xz/xz_dec_lzma2.c:884: undefined reference to `memmove'
make[1]: *** [vmlinuz] Error 1
make: *** [vmlinuz] Error 2

lib/decompress_unxz.c only defines its own memmove function if there is no
macro with the same name.

Signed-off-by: Felix Fietkau <nbd@nbd.name>