dbus_threads_init: call _dbus_threads_init_platform_specific()

dbus_threads_init: call _dbus_threads_init_platform_specific()

This reverses the relationship between these two functions.
Previously, dbus_threads_init() wouldn't allocate dbus_cond_event_tls
on Windows, call check_monotonic_clock on Unix, or call
_dbus_check_setuid on Unix.

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: Ralf Habacker <ralf.habacker@freenet.de>