Clarify purpose of assert in _dl_lookup_symbol_x

Only one of the currently defined flags is incompatible with versioned
symbol lookups, so it makes sense to check for that flag and not its
complement.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Gabriel F. T. Gomes <gabrielftg@linux.ibm.com>
Change-Id: I3384349cef90cfd91862ebc34a4053f0c0a99404