cmake: ensure readline python module target is added before finishing swig.

When the readline target exists (only for non-Android Linux currently),
ensure that target is made a dependency of the finish_swig python-wrap-up
steps.  This ensures it is built when building the lldb target.

Fixes:
https://llvm.org/bugs/show_bug.cgi?id=25038

llvm-svn: 249256