avahi: move libdns_sd compat hook definition inside conditional block

As directed in the buildroot manual, "Optional hooks: keep hook
definition and assignment together in one if block".  And also
to be consistent with the rest of avahi.mk.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>