Use iosfwd instead of iostream

Summary:
The instrumentation header only needs the forward declarations for ostream, so
we can use just include `iosfwd`, as suggested by Riley in an earlier diff.

Reviewed By: kodafb

Differential Revision: D16152451

fbshipit-source-id: 2afbc40e623b180dfc5917fc8093ab15bf647968

ReactCommon/jsi/jsi/instrumentation.h

@@ -6,7 +6,7 @@
  */
 #pragma once
 
-#include <ostream>
+#include <iosfwd>
 #include <string>
 
 #include <jsi/jsi.h>