conf/layer.conf: explicitly depend on networking-layer

conf/layer.conf: explicitly depend on networking-layer

As meta-agl-demo now explicitly depends on meta-flutter layer, it appears
it uses networkmanager package in some of its plugins w/o marking the correct
dependency in its own layer.conf.

Also, cannelloni is no longer a standalone recipe, but a bbappend against
the upstream recipe in meta-networking layer of meta-openembedded repository.

To resolve both of the above issues and make yocto-check-layer happy, let's
add an explicit dependency on networking-layer.

Bug-AGL: SPEC-4685
Change-Id: Ib6781f63bd1f76ccbf2fcd649473bf59407bb7ef
Signed-off-by: Denys Dmytriyenko <denys@konsulko.com>
(cherry picked from commit ebac35dea1de30d02c3ba2d48b91a557f9f0c989)