pom: enable spotbugs plugin

Motivation:
as findbugs is dead move to the successor

https://mailman.cs.umd.edu/pipermail/findbugs-discuss/2017-September/004383.html

Modification:
enable spotbugs plugin that can be activated by:
```
mvn spotbugs:spotbugs
```

Result:
use up-to-date static code analysis tool

Acked-by: Paul Millar
Target: master

Author: kofemann

pom.xml

@@ -160,6 +160,11 @@
               </dependency>
           </dependencies>
       </plugin>
+      <plugin>
+          <groupId>com.github.spotbugs</groupId>
+          <artifactId>spotbugs-maven-plugin</artifactId>
+          <version>3.1.12</version>
+      </plugin>
       <plugin>
           <groupId>org.jacoco</groupId>
           <artifactId>jacoco-maven-plugin</artifactId>