Allow apache-rat-pluin to work when building single modules

The configuration for apache-rat-plugin excludes the '**/target/**' path.
However, when building a single module, the basedir that the plugin uses by
default is the modules' target directory. The exclude signature for target
doesn't match, as it would be attempting to match a target directory under the
target directory itself. This works when building from a multi-module directory,
e.g. brooklyn or brooklyn-server, as the basedir in this case is not the target
directory.

parent/pom.xml

@@ -1090,6 +1090,7 @@
                          <excludes combine.children="merge">
                      so that the child pom entries replace the parent entries
                  -->
+                <basedir>${project.basedir}</basedir>
                 <excludes combine.children="append">
                   <!-- Exclude sandbox because not part of distribution: not in tgz, and not uploaded to maven-central -->
                   <exclude>sandbox/**</exclude>