linphone: add optional dependency on libupnp

linphone can optionally use libupnp, so this dependency should be
accounted for in linphone.mk. In addition, linphone is not compatible
with libupnp18, but misdetects it as a a proper libupnp, causing a
build failure.

The build failure with libupnp18 currently only happens on the next
branch (because libupnp18 has only been added there), but adding the
optional dependency on libupnp makes sense for the master branch
anyway.

Fixes:

  http://autobuild.buildroot.net/results/473c686f9bc5335d25b720cf1b0c45389138a7b4



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>