graph-depends: fix handling of "virtual" in exclude_list

The condition to determine if a virtual package should be excluded
from the list due to "virtual" being passed in --exclude is under a
loop iterating over each entry of the exclude_list, but it doesn't use
the iterator of this list.

Indeed, the condition contains:

	"virtual" in exclude_list

which checks automatically if "virtual" was passed in the list. Due to
this, there is no need for this check to be within the "for p in
exclude_list" iteration. This commit fixes that by moving the check
outside of the loop.

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