Reduce the scope of 'z_erofs_crypto[]' that is not used outside of
'decompressor_crypto.c'.
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202512102025.4mWeBSsf-lkp@xxxxxxxxx/
Signed-off-by: Ferry Meng <mengferry@xxxxxxxxxxxxxxxxx>