[nst] Link against libempathy

[nst] Link against libempathy

As the nautilus-sendto-plugin uses functions from libempathy it should
link directly against it otherwise the build fails when using
-Wl,--as-needed:

.libs/empathy-nautilus-sendto.o: In function
`dup_contact_from_individual':
/tmp/buildd/empathy-3.6.0.1/nautilus-sendto-plugin/empathy-nautilus-sendto.c:73:
undefined reference to `empathy_contact_dup_best_for_action'
/tmp/buildd/empathy-3.6.0.1/nautilus-sendto-plugin/empathy-nautilus-sendto.c:78:
undefined reference to `empathy_contact_can_do_action'