Fix RTCConfiguration equality for SCTP SNAP

Equality comparison for this member was missing, fix it here.

Now SetConfiguration will reject changes to enable_sctp_snap instead of
silently ignoring them. This is aligned with the original author's
intent of not wanting this parameter to be mutable.

Bug: webrtc:426480601
Change-Id: I14e6d39c317e17e99f9ce8deaec9e56755ba5f93
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/498380
Reviewed-by: Philipp Hancke <philipp.hancke@googlemail.com>
Reviewed-by: Guido Urdaneta <guidou@webrtc.org>
Commit-Queue: Joachim Reiersen <joachimr@meta.com>
Cr-Commit-Position: refs/heads/main@{#48476}