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

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

Commit: 365db6b68e3562730048b63de98ef3505efdf9aa
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2017-02-19T22:18:55Z
Releases: 9.2.21
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.

Files