src: use C++ style for struct with initializers

Fixes warning on clang 11:

        In file included from ../../src/node_http2.cc:6:
        ../../src/node_http2.h:508:15: warning: anonymous non-C-compatible type given name for linkage purposes by typedef declaration; add a tag name here [-Wnon-c-typedef-for-linkage]
        typedef struct {
                      ^
                       SessionJSFields
        ../../src/node_http2.h:512:33: note: type is not C-compatible due to this default member initializer
          uint32_t max_invalid_frames = 1000;
                                        ^~~~
        ../../src/node_http2.h:514:3: note: type is given name 'SessionJSFields' for linkage purposes by this typedef declaration
        } SessionJSFields;
          ^

src/node_http2.h

@@ -505,13 +505,13 @@ class Http2Stream::Provider::Stream : public Http2Stream::Provider {
                         void* user_data);
 };
 
-typedef struct {
+struct SessionJSFields {
   uint8_t bitfield;
   uint8_t priority_listener_count;
   uint8_t frame_error_listener_count;
   uint32_t max_invalid_frames = 1000;
   uint32_t max_rejected_streams = 100;
-} SessionJSFields;
+};
 
 // Indices for js_fields_, which serves as a way to communicate data with JS
 // land fast. In particular, we store information about the number/presence