pw_log: Move pw_log proto

Declare log.proto in pw_log rather than pw_log_proto. The log proto is
properly part of pw_log, rather than a standalone module.

Change-Id: I853f28b36a41a2a94564d61ea2e916d9c21076c3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/47220
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>