bird: fix build dependencies

bird: fix build dependencies

The PKG_BUILD_DEPENDS variable must refer to source packages instead of
binary package names.

Fixes the following errors on current OpenWrt master:

Upon scripts/feeds install:

    WARNING: No feed for source package 'libncurses' found
    WARNING: No feed for source package 'libreadline' found

Upon metadata scanning and dependency resolution:

    WARNING: Makefile 'package/feeds/routing/bird/Makefile' has a build dependency on 'libncurses', which does not exist
    WARNING: Makefile 'package/feeds/routing/bird/Makefile' has a build dependency on 'libreadline', which does not exist

Signed-off-by: Jo-Philipp Wich <jo@mein.io>