nvtt plugin -fix for 3.4.0 link error under win32

[mp3butcher]

building 3.4.0 with 3rdParty_VS2017_v141_x64_V11_full.7z under win7
#417

• plugin cmake lacks some external libs
  squish.lib bc7.lib bc6h.lib (from libnvtt repo) should be add to cmake
  Fix:
  i added
  SET(TARGET_EXTERNAL_LIBRARIES ${TARGET_EXTERNAL_LIBRARIES} ${NVTT_LIBRARY} ${NVBC7_LIBRARY} ${NVSQUISH_LIBRARY} ${NVBC6H_LIBRARY} ${NVTHREAD_LIBRARY} ${NVIMAGE_LIBRARY} ${NVCORE_LIBRARY} ${NVMATH_LIBRARY})
  to the plugin TARGET_EXTERNAL_LIBRARIES and it worked at least for Release configuration
  (FindNVTT.cmake might define multi configuration cmake libs definitions...)