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.

src/interfaces/libpq/test/.gitignore

+/uri-regress
+/regress.diff
+/regress.out

src/interfaces/libpq/test/Makefile

@@ -18,5 +18,5 @@ installcheck: all
 		   $(PERL) $(top_srcdir)/$(subdir)/regress.pl
 
 clean distclean maintainer-clean:
-	rm -f $(PROGS)
+	rm -f $(PROGS) *.o
 	rm -f regress.out regress.diff