ligolw: add to the list of ilwd compat errors
Created by: ssachdev12
gwpy fails to read ld style coinc files, specifically tables coinc_inspiral and coinc_event. These fail with errors:
ligo.lw.ligolw.ElementError: line 1972: invalid Column 'coinc_event_id' for Table 'coinc_inspiral'
and
ligo.lw.ligolw.ElementError: line 43: invalid Column 'coinc_def_id' for Table 'coinc_event'
The reading works if I explicitly pass the flag ilwdchar_compat=True. This change will allow reading without having to pass the flag.