Exclude generated ProtobufMessages class from FingBugs report

FindBugs is reporting around 100 warnings in `org.drools.core.marshalling.impl.ProrobufMessages` class , but since the class is generated it does not make sense to fix those. I think the best solution is to exclude the class from the FindBugs report. 

The exclude file is located in src/main/findbugs. I was also considering src/main/resources, but then the findbugs-exclude.xml file would end up in the resulting jar, which is not IMO desirable as it is just developer aid and its not needed at runtime.