cmake: add c-ares::cares as a global visible library

cmake: add c-ares::cares as a global visible library

to address the build failure like:

-- Checking for one of the modules 'libcares'
CMake Error at cmake/modules/BuildBoost.cmake:287 (_add_library):
  _add_library cannot create ALIAS target "c-ares::c-ares" because target
  "c-ares::cares" is imported but not globally visible.
Call Stack (most recent call first):
  cmake/modules/Findc-ares.cmake:31 (add_library)
  src/CMakeLists.txt:344 (find_package)

Signed-off-by: Kefu Chai <tchaikov@gmail.com>