[CMake][libunwind] Define add_target_flags which is missing

It's use was introduced in r353084 but its definition is missing.

Differential Revision: https://reviews.llvm.org/D58745

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@355142 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed