Don't use <sstream> in Streams.h but <iosfwd> instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@32340 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Linker/LinkModules.cpp

@@ -25,6 +25,7 @@
 #include "llvm/Assembly/Writer.h"
 #include "llvm/Support/Streams.h"
 #include "llvm/System/Path.h"
+#include <sstream>
 using namespace llvm;
 
 // Error - Simple wrapper function to conditionally assign to E and return true.