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>
