Exclude "source" features from "tycho-source-plugin" execution

Those features are automatically generated by Tycho and contain the
source bundles of the binary feature they mirror.

It therefore doesn't make any sense to generate source features of the
source features, which currently leads to warnings of the form:

> [WARNING] The following referenced features have missing sources
> org.eclipse.draw2d.source_3.22.0.202411052148
> org.eclipse.gef.source_3.22.0.202411052148
> org.eclipse.zest.source_3.22.0.202411052148

pom.xml

@@ -89,6 +89,9 @@
 						<plugin id="org.eclipse.gef.doc.isv" />
 						<plugin id="org.eclipse.zest.doc.isv" />
 						<!-- also possible to exclude feature-->
+						<feature id="org.eclipse.draw2d.source" />
+						<feature id="org.eclipse.gef.source" />
+						<feature id="org.eclipse.zest.source" />
 					  </excludes>
 					</configuration>
 				  </execution>