[3.0.x] vlc_common: remove warnings on VLC_OBJECT

Fix an infinite amount of warnings on Darwin:

../../include/vlc_variables.h:563:5: warning: due to lvalue conversion of the controlling expression, association of type 'const struct vlc_common_members' will never be selected because it is qualified [-Wunreachable-code-generic-assoc]
    var_Create( p_obj, psz_name, VLC_VAR_STRING | VLC_VAR_DOINHERIT
    ^
../../include/vlc_variables.h:122:39: note: expanded from macro 'var_Create'
#define var_Create(a,b,c) var_Create( VLC_OBJECT(a), b, c )
                                      ^
../../include/vlc_common.h:481:15: note: expanded from macro 'VLC_OBJECT'
        const struct vlc_common_members: (const vlc_object_t *)(&(x)->obj) \

It does not backport the 4.0 way of marking objects.