Make src/interfaces/libpq/test clean up after itself.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 090f21bbad98001979da8589e9647a1d49bce4ee
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2017-02-19T22:18:10Z
Releases: 10.0
Make src/interfaces/libpq/test clean up after itself.

It failed to remove a .o file during "make clean", and it lacked
a .gitignore file entirely.

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