Emit additional headers in generated .h file instead of .cc

For Flatbuffers compatibility.

From what I can tell, File::additional_headers is not used by gRPC
itself or its default protobuf implementation; it was added for
Flatbuffers support (it just returns "" for protobuf).

In the Flatbuffer case, the generated header contains references to
Flatbuffer gRPC glue code which is in a header in additional_headers.
Prior to this patch, this meant that the generated .h file could not
be included unless this glue file was included first.

Because the protobuf implementation of additional_headers returns
an empty string, I think this change should be safe to do and not
have unintentional consequences.