[lldb] [Python] Build readline override module only on Linux

Restrict building the readline override to Linux only.  It both does not
build on *BSD systems, and is largely irrelevant since they default to
using libedit over readline anyway.  This restores the behavior
of the old readline override that also was built only on Linux.

Differential Revision: https://reviews.llvm.org/D69846

Author: mgorny

lldb/source/Plugins/ScriptInterpreter/Python/PythonReadline.h

@@ -9,7 +9,7 @@
 #ifndef LLDB_PLUGINS_SCRIPTINTERPRETER_PYTHON_PYTHONREADLINE_H
 #define LLDB_PLUGINS_SCRIPTINTERPRETER_PYTHON_PYTHONREADLINE_H
 
-#if !defined(LLDB_DISABLE_LIBEDIT) && !defined(__APPLE__)
+#if !defined(LLDB_DISABLE_LIBEDIT) && defined(__linux__)
 // NOTE: Since Python may define some pre-processor definitions which affect the
 // standard headers on some systems, you must include Python.h before any
 // standard headers are included.