Always initialize threading before allocating a dynamic mutex

Always initialize threading before allocating a dynamic mutex

Dynamic allocation of mutexes can fail anyway, so this is easy.

Justification for not keeping the dummy mutex code-paths, even as an
opt-in thing for processes known to be high-performance and
single-threaded: real mutexes only cut the throughput of
test/dbus-daemon.c by a couple of percent on my laptop (from around
6700 to around 6600 messages per second), and libdbus crashes caused
by not calling dbus_threads_init_default() are sufficiently widespread
that they're wasting a lot of everyone's time.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=54972
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Alban Crequy <alban.crequy@collabora.co.uk>
Reviewed-by: Anas Nashif <anas.nashif@intel.com>