zstream: remove duplicate highbit64 definition

When building a static build (--disable-shared), zstream fails to link
because of the duplicate highbit64() in libzpool/kernel.c. Since they're
identical, and the libzpool one is visible to zstream, we remove
zstream's copy and just use the common one.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16426