Add missing uninstallation rule for BackgroundPsql.pm

Commit a4c17c8 added in the install rule but not the uninstall
rule.

Author: petere

src/test/perl/Makefile

@@ -31,6 +31,7 @@ uninstall:
 	rm -f '$(DESTDIR)$(pgxsdir)/$(subdir)/PostgreSQL/Test/SimpleTee.pm'
 	rm -f '$(DESTDIR)$(pgxsdir)/$(subdir)/PostgreSQL/Test/RecursiveCopy.pm'
 	rm -f '$(DESTDIR)$(pgxsdir)/$(subdir)/PostgreSQL/Test/Cluster.pm'
+	rm -f '$(DESTDIR)$(pgxsdir)/$(subdir)/PostgreSQL/Test/BackgroundPsql.pm'
 	rm -f '$(DESTDIR)$(pgxsdir)/$(subdir)/PostgreSQL/Version.pm'
 
 endif