userguide: run tex twice

.. to generate proper cross-references.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/Makefile b/Makefile
index 9e5cfa7..7f6c891 100644
--- a/Makefile
+++ b/Makefile
@@ -9,6 +9,7 @@
 
 userguide.pdf: userguide/userguide.tex
 	$(tex) $(tflags) $^
+	$(tex) $(tflags) $^
 
 userguide/userguide.tex: rest-api.tex host-management.tex console.tex code-update.tex