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
diff --git a/tests_reg/test/secure_fw/suites/fwu/mcuboot/secure/CMakeLists.txt b/tests_reg/test/secure_fw/suites/fwu/mcuboot/secure/CMakeLists.txt
index 5d84ff6..4f45d97 100644
--- a/tests_reg/test/secure_fw/suites/fwu/mcuboot/secure/CMakeLists.txt
+++ b/tests_reg/test/secure_fw/suites/fwu/mcuboot/secure/CMakeLists.txt
@@ -39,6 +39,6 @@
 )
 
 target_link_libraries(tfm_s_tests
-    INTERFACE
+    PRIVATE
         tfm_test_suite_fwu_s
 )