Disabled warning C4251 on MSVC builds.

See http://reviews.llvm.org/D3934 for more details.
This is only the CMakeLists.txt portion of that change.

Change by Zachary Turner.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@209756 91177308-0d34-0410-b5e6-96231b3b80d8