[CMake] Include CMakeDependentOption in libunwind

This should resolve the breakage introduced in r337867 which introduced
the use of cmake_dependent_option without include the necessary file.

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@337868 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed