docs: schemas: Fix missing timestamp inside backingStore

All backingStore XML definitions have a XML tag with the timestamp. This
timestamp is not defined insinde RNG volume storage schema and it is
causing some problems to validate and check volume XMLs.

https://bugzilla.redhat.com/show_bug.cgi?id=1594266Signed-off-by: NJulio Faracco <jcfaracco@gmail.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

docs/schemas/storagevol.rng

@@ -139,6 +139,7 @@
         </element>
         <ref name='format'/>
         <ref name='permissions'/>
+        <ref name='timestamps'/>
       </interleave>
     </element>
   </define>