libelementary: depends on libethumb

This was exposed because I still had an old libethumb in my staging
directory so it was detected by configure, but because of the missing
dependency it was still the (incompatible) version from before the
1.7.4 bump.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>