Fix imstkFind.cmake

Ensure that a library is not created multiple times. Otherwise you may get
the following error:
CMake Error at .../iMSTK-build/install/lib/cmake/iMSTK-2.0/modules/imstkFind.cmake:192 (add_library):
  add_library cannot create imported target "g3log" because another target
  with the same name already exists.

or:

CMake Error at .../iMSTK-build/install/lib/cmake/iMSTK-2.0/modules/Findg3log.cmake:17 (target_link_libraries):
  Cannot specify link libraries for target "g3log" which is not built by this
  project.