asterisk: add dependency to asterisk-chan-rtp

asterisk: add dependency to asterisk-chan-rtp

This commit makes asterisk-chan-rtp depend on
asterisk-res-rtp-multicast. Without the res-rtp-multicast module
chan-rtp cannot be loaded:

OpenWrt*CLI> module load chan_rtp.so
Unable to load module chan_rtp.so
Command 'module load chan_rtp.so' failed.
[Jan  9 08:51:01] ERROR[16337]: loader.c:281 module_load_error: Error loading module 'chan_rtp.so': Error relocating /usr/lib/asterisk/modules/chan_rtp.so: ast_multicast_rtp_create_options: symbol not found

This issue was mentioned in the OpenWrt forum ([1]).

[1] https://forum.openwrt.org/t/asterisk-makes-a-problem-with-loading/84816

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