Fix bad variant usage in kernel dependencies
    
    This was resulting in a variant build with dependencies like so:
    
    error: Failed dependencies:
    	kernel--vanilla-core-uname-r = ...
    
    The variant already has a - at the start of it, so the always-there one
    should be after variant and variant will provide the one between kernel
    and itself.
    
    Signed-off-by: Jarod Wilson <jarod@redhat.com>