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) \