This library is used by other packages, so should be in the lib output.
By removing unused sections, libecpg will not contain any references to other
outputs and thus does not increase the closure for the lib output anymore.
This will also help massively when splitting a dev output later.
As a side-effect, this also unbreaks pkgsMusl.postgresql_12_jit and
pkgsMusl.postgresql_13_jit. For, at least to me, unknown reasons, those build
fine now.
