add missing API

SkMemoryWStream needs SK_API to be accessible from PDFium.

R=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1808683002

Review URL: https://codereview.chromium.org/1808683002

include/core/SkStream.h

@@ -370,7 +370,7 @@ private:
     typedef SkWStream INHERITED;
 };
 
-class SkMemoryWStream : public SkWStream {
+class SK_API SkMemoryWStream : public SkWStream {
 public:
     SkMemoryWStream(void* buffer, size_t size);
     bool write(const void* buffer, size_t size) override;