support/dependencies: add check for c++ multilib

support/dependencies: add check for c++ multilib

Some packages build C++ 32bits host-tools and need the g++-multilib to
be installed on the build machine. As example, qt5webengine builds a C++
host-tool when target is 32bits.

Add the check for g++-multilib to the dependencies script; and update
the Dockerfile to install g++-multilib package.

Signed-off-by: Gaël PORTAY <gael.portay@savoirfairelinux.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>