graph-depends: avoid use of global var 'rule' in get_depends

graph-depends: avoid use of global var 'rule' in get_depends

Function get_depends was recently changed to support both normal
dependencies as reverse dependencies, via a global variable 'rule' that
equals 'show-depends' or 'show-rdepends'.

As a subsequent function will extract this function get_depends to a
separate file, the use of globals is problematic.

Instead, pass the global as an argument.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>