message: Fix jsoncpp include issue in non-superbuild mode
diff: include_directories(${PROJECT_SOURCE_DIR}/core)
  include_directories(${PROJECT_SOURCE_DIR}/third_party/mavlink/include)
  +find_package(JsonCpp REQUIRED)
  +
  add_executable(unit_tests_runner
  ${UNIT_TEST_SOURCES}
  )
  @@ -17,6 +19,7 @@ target_link_libraries(unit_tests_runner
  mavsdk_camera
  mavsdk_calibration
  CURL::libcurl
  + JsonCpp::jsoncpp
  gtest
  gtest_main
  gmock
  