For #214, as observed by Thibaud Michaud.

* spot/twa/acc.hh: Name the anonymous struct.
* spot/twa/acc.hh, spot/twa/acc.cc, spot/parseaut/parseaut.yy,
spot/twaalgos/dtwasat.cc, spot/twaalgos/remfin.cc,
spot/twaalgos/sepsets.cc, spot/twaalgos/totgba.cc: Adjust all usages.
* NEWS: Mention the renaming.