bird: Update to version 2.15.1

bird: Update to version 2.15.1

- Update from version 2.14 to 2.15.1
- Update of rootfile not required
- Changelog
    2.15.1
	  o OSPF: Fix regression in handling PtP links
	  o RPKI: Handle connection resets properly
	  o Static: Reject invalid combination of options
	  o Fix builds with limited set of protocols
    2.15
	  o BGP: Send hold timer
	  o BGP: New options to specify required BGP capabilities
	  o BFD: Improvements to 'show bfd sessions' command
	  o RPKI: New 'local address' configuration option
	  o Linux: Support for more route attributes, including
	    TCP congestion control algorithm
	  o Support for UDP logging
	  o Static routes can have both nexthop and interface specified
	  o Completion of command options in BIRD client
	  o Many bugfixes and improvements

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>