package/leveldb: turn snappy into an optional dependency

snappy is not a mandatory dependency to build leveldb. Back when it
was introduced in Buildroot, as of version 1.18, the build logic
already made snappy an optional dependency.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>