toolchain-external: select netbsd-queue for musl toolchains

Following the introduction of the check that target packages must have
their Config.in option enabled, we started to see failures related to
netbsd-queue. Yann fixed the internal toolchain case in commit
e84fd04e. This commit fixes the
similar issue, but for the external toolchain case.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>