Export OFStreamBase in ostream.h

GCC 4.9.2 on MIPS generates a reference to OFStreamBase()
d8.cc. In debug mode OFStreamBase is local to libv8_base and
linking fails.

Change-Id: I93bb93d03a4cc81c59f94cf2168c92557845e87d
Reviewed-on: https://chromium-review.googlesource.com/c/1258903Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Ivica Bogosavljevic <ibogosavljevic@wavecomp.com>
Cr-Commit-Position: refs/heads/master@{#56413}

src/ostreams.h

@@ -18,7 +18,7 @@
 namespace v8 {
 namespace internal {
 
-class OFStreamBase : public std::streambuf {
+class V8_EXPORT_PRIVATE OFStreamBase : public std::streambuf {
  public:
   explicit OFStreamBase(FILE* f);
   ~OFStreamBase() override = default;