Re-add tests/include path for test helpers

These must include from both the framework and the main repository since
the test helpers straddle the two locations.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f915188..64d95dc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -418,6 +418,7 @@
     set_base_compile_options(mbedtls_test_helpers)
     target_include_directories(mbedtls_test_helpers
         PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/framework/tests/include
+        PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/tests/include
         PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/include
         PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/tf-psa-crypto/include
         PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/tf-psa-crypto/drivers/builtin/include