cmake: fix OpenCL-CTS support (#806)

OpenCL-CTS depends on SPIRV-Headers since KhronosGroup/OpenCL-CTS@555b7cd

Author: rjodinchr

tests/conformance/CMakeLists.txt

@@ -17,6 +17,7 @@ set(CLConform_LIBRARIES -lOpenCL)
 set(CL_LIBCLCXX_DIR dummy)
 set(CL_INCLUDE_DIR ${CLVK_PROJECT_SOURCE_DIR}/external/OpenCL-Headers)
 set(CL_LIB_DIR ${CLVK_PROJECT_SOURCE_DIR}/external/OpenCL-ICD-Loader/build)
+set(SPIRV_INCLUDE_DIR "${SPIRV_HEADERS_SOURCE_DIR}")
 set(VULKAN_IS_SUPPORTED ON)
 
 option(CLVK_BUILD_CONFORMANCE_TESTS_GL_GLES_SUPPORTED