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;
          ^

PR-URL: https://github.com/nodejs/node/pull/32134


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>