Fix client-only installation

The psql Makefile was not creating $(datadir) before installing
psqlrc.sample there.

In most cases, the directory would be created in some other way, but for
the documented from-source client-only installation procedure, it could
fail.
Reported-by: Mike Blackwell <mike.blackwell@rrd.com>

src/bin/psql/Makefile

@@ -57,7 +57,7 @@ install: all installdirs
 	$(INSTALL_DATA) $(srcdir)/psqlrc.sample '$(DESTDIR)$(datadir)/psqlrc.sample'
 
 installdirs:
-	$(MKDIR_P) '$(DESTDIR)$(bindir)'
+	$(MKDIR_P) '$(DESTDIR)$(bindir)' '$(DESTDIR)$(datadir)'
 
 uninstall:
 	rm -f '$(DESTDIR)$(bindir)/psql$(X)' '$(DESTDIR)$(datadir)/psqlrc.sample'