fix: register geos codecs in postgis example main.go

[sebdotv]

This fix is required to make main.go work.

Without it the following error is raised, even though main_test.go works fine:

go run examples/postgis/main.go --write
can't scan into dest[2]: cannot scan unknown type (OID 20937) in text format into **geos.Geom
exit status 1

[twpayne]

Thanks, the fix is also needed for -read, but I'll merge this PR and fix that.

[twpayne]

Patched in 4a39130. Thank you for the spot and fix!