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 ec2b18f..ca79882 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -393,6 +393,7 @@
     add_library(mbedtls_test_helpers OBJECT ${MBEDTLS_TEST_HELPER_FILES})
     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}/library
         PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/3rdparty/everest/include)