Fix bash completion installation

Currently this fails if the directory for the completion does not
already exist, at least with FreeBSD install(1).

Makefile

@@ -26,6 +26,7 @@ install: wlopm
 	install wlopm $(DESTDIR)$(BINDIR)
 	install -d $(DESTDIR)$(MANDIR)/man1
 	install -m 644 wlopm.1 $(DESTDIR)$(MANDIR)/man1
+	install -d $(DESTDIR)$(BASHCOMPDIR)
 	install bash-completion $(DESTDIR)$(BASHCOMPDIR)/wlopm
 
 uninstall: