[infrastructure] List spotless plugin in plugins section, not only pl…

…uginManagement secition (openhab#6867)

Signed-off-by: Arne Seime <arne.seime@gmail.com>
Signed-off-by: Hans-Reiner Hoffmann <hans-reiner.hoffmann@gmx.de>

pom.xml

@@ -654,6 +654,10 @@ Import-Package: \\
           </execution>
         </executions>
       </plugin>
+      <plugin>
+        <groupId>com.diffplug.spotless</groupId>
+        <artifactId>spotless-maven-plugin</artifactId>
+      </plugin>
     </plugins>
     <extensions>
        <extension>