fix build error with the latest clang
fix build error with the latest clang

Summary:
```lang=bash
wangle/ssl/SSLCacheProvider.h:40:5: error: unsupported: typedef changes linkage of anonymous type, but linkage was already computed
  } CacheContext;
    ^
wangle/ssl/SSLCacheProvider.h:33:17: note: use a tag name here to establish linkage prior to definition
  typedef struct {
                ^
                 CacheContext
1 error generated.
```

Reviewed By: meyering

Differential Revision: D6023170

fbshipit-source-id: 984613bdf9abd8232849067201834d46b3aa8cfb

facebook/wangle,facebook/wangle,facebook/wangle