Set LIBPROTOBUF_EXPORT on GzipOutputStream::Options

This fixes issue #2610.

This was originally fixed in a101fa5289 , but got
reverted by d36c0c538a

src/google/protobuf/io/gzip_stream.h

@@ -118,7 +118,7 @@ class LIBPROTOBUF_EXPORT GzipOutputStream : public ZeroCopyOutputStream {
     ZLIB = 2,
   };
 
-  struct Options {
+  struct LIBPROTOBUF_EXPORT Options {
     // Defaults to GZIP.
     Format format;