Update CMakeLists.txt as per new file RegisterContextDummy.cpp



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@180061 91177308-0d34-0410-b5e6-96231b3b80d8
Daniel Malea reported seeing warnings for the use of anonymous namespaces in our public API.
Removing these namespace { ... } declarations (but still keeping the helper *Impl objects outside of namespace lldb proper)

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@180067 91177308-0d34-0410-b5e6-96231b3b80d8