hkdf_test binary should only have one icp instance

hkdf_test binary should only have one icp instance

The build for test binary hkdf_test was linking both against libicp
and libzpool. This results in two instances of libicp inside the
binary but the call to icp_init() only initializes one of them!

Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #8850