gmain: close pidfd when finalizing GChildWatchSource

A file-descriptor was created with the introduction of pidfd_getfd() but
nothing is closing it when the source finalizes. The GChildWatchSource is
the creator and consumer of this FD and therefore responsible for closing
it on finalization.

The pidfd leak was introduced in !2408.

This fixes issues with Builder where anon_inode:[pidfd] exhaust the
available FD limit for the process.

Fixes #2708