fix uninitialized variable EXTRA_COMPONENT_DIRS

tools/unit-test-app/CMakeLists.txt

@@ -2,10 +2,8 @@
 # CMakeLists in this exact order for cmake to work correctly
 cmake_minimum_required(VERSION 3.5)
 
-set(EXTRA_COMPONENT_DIRS
-    ${EXTRA_COMPONENT_DIRS}
-    "$ENV{IDF_PATH}/examples/cxx/experimental/experimental_cpp_component/"
-    "$ENV{IDF_PATH}/examples/peripherals/rmt/ir_protocols/components")
+list(APPEND EXTRA_COMPONENT_DIRS "$ENV{IDF_PATH}/examples/cxx/experimental/experimental_cpp_component/"
+                                 "$ENV{IDF_PATH}/examples/peripherals/rmt/ir_protocols/components")
 
 include($ENV{IDF_PATH}/tools/cmake/project.cmake)
 project(unit-test-app)