giscanner: Look for _finish functions in constructors too

async _new functions get treated as static methods, while their finish functions get treated as constructors.

That leads to g-ir-scanner unable to match them up, causing warnings like this for gdbus codegen'd code:

Warning: Namespace: Couldn't find 'new_finish' for the corresponding async function: 'new'

This commit merges static methods and constructors together for the purposes of finding async/finish pairs.