schema: make target mandatory for filesystems

Commit 12967c3e13 incorrectly made the target optional.
The validation was re-introduced by commit e8863b91f (which
put it into FSDefValidate instead of the parser), but
it did not amend the schema to make it mandatory again.

https://bugzilla.redhat.com/show_bug.cgi?id=1969232

Fixes: e8863b91fb
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>

docs/schemas/domaincommon.rng

@@ -2859,12 +2859,10 @@
           </group>
         </choice>
         <interleave>
-          <optional>
-            <element name="target">
-              <attribute name="dir"/>
-              <empty/>
-            </element>
-          </optional>
+          <element name="target">
+            <attribute name="dir"/>
+            <empty/>
+          </element>
           <optional>
             <attribute name="accessmode">
               <choice>