include checkstyle-suppressions.xml file in source distribution

Now checkstyle can be successfully executed from the source distribution (e.g. as part of the site life-cylce).

Author: PascalSchumacher

src/assembly/src.xml

@@ -26,6 +26,7 @@
             <includes>
                 <include>.travis.yml</include>
                 <include>checkstyle.xml</include>
+                <include>checkstyle-suppressions.xml</include>
                 <include>findbugs-exclude-filter.xml</include>
                 <include>LICENSE.txt</include>
                 <include>NOTICE.txt</include>