[lldb/Host] Use Host/Config.h entries instead of a global define.

As suggested by Pavel in a code review:

> Can we replace this (and maybe python too, while at it) with a
> Host/Config.h entry? A global definition means that one has to
> recompile everything when these change in any way, whereas in
> practice only a handful of files need this..

Differential revision: https://reviews.llvm.org/D71280
diff --git a/lldb/source/API/SystemInitializerFull.cpp b/lldb/source/API/SystemInitializerFull.cpp
index 0acc496..ffb302d 100644
--- a/lldb/source/API/SystemInitializerFull.cpp
+++ b/lldb/source/API/SystemInitializerFull.cpp
@@ -7,8 +7,8 @@
 //===----------------------------------------------------------------------===//
 
 #include "SystemInitializerFull.h"
-
 #include "lldb/API/SBCommandInterpreter.h"
+#include "lldb/Host/Config.h"
 
 #if !defined(LLDB_DISABLE_PYTHON)
 #include "Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.h"