MIPS: boot/compressed/: add __bswapdi2() to target for ZSTD decompression

For MIPS pre-boot, when CONFIG_KERNEL_ZSTD=y, the decompressor
function uses __bswapdi2(), so this object file should be added to
the target object file.

Fixes these build errors:

mips-linux-ld: arch/mips/boot/compressed/decompress.o: in function `xxh64':
decompress.c:(.text+0x8be0): undefined reference to `__bswapdi2'
mips-linux-ld: decompress.c:(.text+0x8c78): undefined reference to `__bswapdi2'
mips-linux-ld: decompress.c:(.text+0x8d04): undefined reference to `__bswapdi2'
mips-linux-ld: arch/mips/boot/compressed/decompress.o:decompress.c:(.text+0xa010): more undefined references to `__bswapdi2' follow

Fixes: 0652035a ("asm-generic: unaligned: remove byteshift helpers")
Fixes: cddc40f5 ("mips: always link byteswap helpers into decompressor")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>