init: statically link libselinux to init_tests

The shared libselinux library does not export all of the symbols that
we use in init and the linker is now complaining about this, so let's
use the static libselinux library in init_tests to match init itself.

Test: build, init unit tests
Change-Id: I9011a959a7c49446b3529740e606140a4ee8c32d