asterisk: add res_timing_timerfd to base package

asterisk: add res_timing_timerfd to base package

This is a response to [1] and [2]. In short, since upstream commit
"channel.c: Resolve issue with receiving SIP INFO packets for DTMF" a
segmentation fault occurs during DTMF when no timing module is loaded.

This commit adds the timerfd timing module to the asterisk base package.
Other possibilities would have been res_timing_pthread and
res_timing_dahdi, but timerfd is the obvious choice because it's
efficient and OpenWrt offers the required kernel support (see [3] for
more details).

Note: even without any timing module the segmentation fault should not
occur according to upstream. Upstream is still working on that. But in
general it makes sense to have a timing module available (loaded) and
the module is tiny anyway.

[1] https://github.com/openwrt/telephony/issues/597
[2] https://issues.asterisk.org/jira/browse/ASTERISK-28800
[3] https://wiki.asterisk.org/wiki/display/AST/Timing+Interfaces

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>