res_pjsip: Actually enable session timers when timers=always

When a pjsip endpoint is defined with timers=always, this has been a
functional noop.  This patch correctly sets the feature bitmap to both
enable support for session timers and to enable them even when the
endpoint itself does not request or support timers.

ASTERISK-29603
Reported-By: Ray Crumrine

Change-Id: I8b5eeaa9ec7f50cc6d96dd34c2b4aa9c53fb5440