Fix BULLET_INCLUDE_DIRS in DARTCoreConfig.cmake
Fix BULLET_INCLUDE_DIRS in DARTCoreConfig.cmake

The fix from #352 doesn't work if the downstream code
doesn't have the HAVE_BULLET_COLLISION variable defined.
This change just inserts the BULLET_INCLUDE_DIRS
variable into DARTCore_INCLUDE_DIRS
and trusts that it will be blank and not affect
anything if it is unset.

Closes #351.

dartsim/dart,dartsim/dart,dartsim/dart,dartsim/dart,axeisghost/DART6motionBlur,axeisghost/DART6motionBlur,dartsim/dart,axeisghost/DART6motionBlur