Enables buddy class loading

Adds the buddy policy 'registered' to the
manifest so that custom appenders provided
by OSGi bundles can be loaded via buddy
class loading.

The providing bundle must define a
corresponding Eclipse-RegisterBuddy header.

Closes qos-ch#72

pom.xml

@@ -331,6 +331,7 @@
             </Import-Package>
             <Bundle-DocURL>https://reload4j.qos.ch/</Bundle-DocURL>
             <Bundle-SymbolicName>org.apache.log4j</Bundle-SymbolicName>
+            <Eclipse-BuddyPolicy>registered</Eclipse-BuddyPolicy>
           </instructions>
         </configuration>
       </plugin>