Fix Windows build break related to MainLoop.

[amccarth]

Since this is a trivial build fix that affects only Windows, I'm going to commit.

[labath]

I am sorry for the breakage. I think changes like these are ok to commit without review. We can always change it later if there is something wrong.

include/lldb/Host/MainLoop.h

 //===-- MainLoop.h ----------------------------------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef lldb_Host_MainLoop_h_
 #define lldb_Host_MainLoop_h_
 
 #ifdef _WIN32
+#include "lldb/Host/MainLoopBase.h"
 namespace lldb_private
 {
 typedef MainLoopBase MainLoop;
 }
 #else
 #include "lldb/Host/posix/MainLoopPosix.h"
 namespace lldb_private
 {
 typedef MainLoopPosix MainLoop;
 }
 #endif
 
 #endif // lldb_Host_MainLoop_h_