fix: add find_package(Boost) to eagleye_gnss_converter

[SakodaShintaro]

Some people have encountered the following errors.

Starting >>> eagleye_gnss_converter
--- stderr: eagleye_gnss_converter                           
CMake Error at /home/user/autoware/install/ament_cmake_auto/share/ament_cmake_auto/cmake/ament_auto_add_executable.cmake:66 (add_executable):
  Target "gnss_converter" links to target "Boost::system" but the target was
  not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?
Call Stack (most recent call first):
  CMakeLists.txt:23 (ament_auto_add_executable)


CMake Error at /home/user/autoware/install/ament_cmake_auto/share/ament_cmake_auto/cmake/ament_auto_add_executable.cmake:66 (add_executable):
  Target "gnss_converter" links to target "Boost::thread" but the target was
  not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?
Call Stack (most recent call first):
  CMakeLists.txt:23 (ament_auto_add_executable)

It seems that this issue will be fixed by the change in this pull request.