[CMake] Need to set WITH_LOCKDOWN on debugserver target

Turns out WITH_LOCKDOWN define changes the struct layout and constructor implementation for RNBSocket which is used in debugserver.cpp, so we need to make sure this is consistent.

In the future we should change WITH_LOCKDOWN to be configured in a generated header, but for now we can just set it correctly.

<rdar://problem/33900552>

llvm-svn: 312666