Don't build LLDBWrapPython.cpp for mingw.

Previous check relied on -DLLDB_DISABLE_PYTHON which was not valid as
it is defined in the top level LLDB Makefile which is included after the check.
If this check is moved after the inclusion of top level Makefile then 
BUILT_SOURCES is not properly handled. So I am using the scheme present
in the Host/Makefile.

llvm-svn: 204459