support/graph-depends: fix out-of-tree usage

graph-depends calls make to get the list of packages, and the
dependencies of each package.

When called out-of-tree, the Makefile is a wrapper that calls
the real Makefile, so make will spit out a line like:
  make -C /path/to/buildroot O=/path/to/build-dir show-targets

which graph-depends wrongly believes is part of the target list.

Be silent when calling make, as we really only want the target
and dependency lists.

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