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

tests/zfs-tests/tests/functional/hkdf/Makefile.am

@@ -2,8 +2,7 @@ include $(top_srcdir)/config/Rules.am
 
 AM_CPPFLAGS += -I$(top_srcdir)/include
 AM_CPPFLAGS += -I$(top_srcdir)/lib/libspl/include
-LDADD = $(top_srcdir)/lib/libicp/libicp.la
-LDADD += $(top_srcdir)/lib/libzpool/libzpool.la
+LDADD = $(top_srcdir)/lib/libzpool/libzpool.la
 
 AUTOMAKE_OPTIONS = subdir-objects