fixxref: Add support for ‘-es’ plurals in the poor man’s plural support

We currently try to link symbols which end in ‘-s’ as a way of handling plurals. Extend that to plurals which end in ‘-es’, which can happen in English if the singular ends in an ‘s’. For example, ‘GNetworkAddress’ → ‘GNetworkAddresses’.

Signed-off-by: Philip Withnall withnall@endlessm.com