src/Makefile.am: Add forgotten backslash
    
    As I've pushed 5892944f I haven't noticed one small nitpick.
    There was this backslash missing on the line 1231 in the
    enumeration of libraries to be added to vbox storage driver. This
    resulted in nondeterministic build which sometimes succeeded and
    sometimes failed.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>