[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

llvm-svn: 355142
GitOrigin-RevId: dc1b8e9f4478c838c8704295d97f5e514edb9cd0
1 file changed