SignalHandler: add a missing std::move()

Connections are movable, and it's more efficient to move them than to
copy (which ups/downs an atomic ref count), so do a move here.

Amends acf7f0b1ae956f2fac7182c194e0441cd9c6f4d0 (5.4).

Pick-to: 6.11 6.10 6.8
Change-Id: I5d71f7c0395539110901629cb6c667fc892870d7
Reviewed-by: Arno Rehn <a.rehn@menlosystems.com>