[JSONExporter] Try to appease buildbot. NFC.

The compiler does not seem to able move a local variable in the
function's return statement.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@338466 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Exchange/JSONExporter.cpp b/lib/Exchange/JSONExporter.cpp
index 7e77280..423768b 100644
--- a/lib/Exchange/JSONExporter.cpp
+++ b/lib/Exchange/JSONExporter.cpp
@@ -173,7 +173,7 @@
   }
 
   root["statements"] = std::move(Statements);
-  return root;
+  return json::Value(std::move(root));
 }
 
 static void exportScop(Scop &S) {