Set spdlog submodule branch to "master" explicitly. (#4087)
The default branch for the spdlog repository on GitHub recently changed from "master"
to "v1.x", which has a different API for `syslog_sink::syslog_sink()`. This breaks
builds of the server for anyone who has checked out the submodules since that change.
Fixes #4077.
Co-authored-by: Derek Murray <demurra@microsoft.com>
