Explicitly initialize the base class of LogStream

Summary:
The standard does not define that `std::ostream` should have a default constructor, so this was invalid and was an error on Windows.
This explicitly initializes the base class by passing a `nullptr`.

Reviewed By: simpkins

Differential Revision: D5259690

fbshipit-source-id: b8914d73ff2682e1a4447b8338860259778b2247

folly/experimental/logging/LogStream.cpp

@@ -46,7 +46,7 @@ LogStreamBuffer::int_type LogStreamBuffer::overflow(int_type ch) {
   }
 }
 
-LogStream::LogStream() {
+LogStream::LogStream() : std::ostream(nullptr) {
   rdbuf(&buffer_);
 }