Fix hioOpenVDB build on windows

[hshakula]

Description of Change(s)

Add public interface to hioOpenVDB library.

Fixes Issue(s)

CMake error on trying to link against USD that is built with OpenVDB support:

CMake Error at D:/amd-gpuopen/BlenderUSDHydraAddon/bin/bin-21.08/USD/install/cmake/pxrTargets.cmake:540 (message):
  The imported target "hioOpenVDB" references the file

     "D:/amd-gpuopen/BlenderUSDHydraAddon/bin/bin-21.08/USD/install/lib/hioOpenVDB.lib"

  but this file does not exist.  Possible reasons include:

  * The file was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and contained

     "D:/amd-gpuopen/BlenderUSDHydraAddon/bin/bin-21.08/USD/install/cmake/pxrTargets.cmake"

  but not all the files it references.

Call Stack (most recent call first):
  D:/amd-gpuopen/BlenderUSDHydraAddon/bin/bin-21.08/USD/install/pxrConfig.cmake:19 (include)
  cmake/defaults/Packages.cmake:37 (find_package)
  CMakeLists.txt:11 (include)

Due to the fact that hioOpenVDB had no exported symbols, hioOpenVDB.lib was not generated thus causing such an error.

[jilliene]

Filed as internal issue #USD-6809