FWU: Fix TEST_S_FWU build

When TEST_S_FWU are built alone (-DTEST_S_FWU=ON) then none tests are run.
This is because tfm_test_suite_fwu_s is linked as INTERFACE which is not
correct. Fix this by using PRIVATE linkage.

Signed-off-by: Bohdan Hunko <Bohdan.Hunko@infineon.com>
Change-Id: I685278987d748bd8c73f01c35f9a27a7f27f5b29
1 file changed