Add src folder to the maven-clean-plugin config.

The content is generated at build time so it has to be cleaned.
diff --git a/pom.xml b/pom.xml
index d8cf11b..b2281f1 100644
--- a/pom.xml
+++ b/pom.xml
@@ -58,6 +58,13 @@
         <configuration>
           <filesets>
             <fileset>
+              <directory>src</directory>
+              <includes>
+                <include>**/*</include>
+              </includes>
+              <followSymlinks>false</followSymlinks>
+            </fileset>
+            <fileset>
               <directory>@dot</directory>
               <includes>
                 <include>**/*</include>