tmf: Fix IDE error in o.e.tc.tmf.core.config.exsd

This patch fixes the error below, reported by the Eclipse IDE:
Element 'attribute' with use set to 'default' must have the attribute
'value' defined.

Change-Id: Idc737f6c4dccc9b83ecc1e4c302f76b627babfe2
Signed-off-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/c/tracecompass/org.eclipse.tracecompass/+/205390
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Trace Compass Bot <tracecompass-bot@eclipse.org>
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>

tmf/org.eclipse.tracecompass.tmf.core/schema/org.eclipse.tracecompass.tmf.core.config.exsd

@@ -56,7 +56,7 @@
                </documentation>
             </annotation>
          </attribute>
-         <attribute name="class" type="string" use="default">
+         <attribute name="class" type="string" use="required">
             <annotation>
                <documentation>
                   The fully qualified name of a class that implements the &lt;samp&gt;IConfigurationSource&lt;/samp&gt; interface.