[mail] Fix UnsupportedDataTypeException (#12568)

Also-by: Jan N. Klug <jan.n.klug@rub.de>
Signed-off-by: Hans-Jörg Merk <github@hmerk.de>

bundles/org.openhab.binding.mail/pom.xml

@@ -33,4 +33,26 @@
     </dependency>
   </dependencies>
 
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.codehaus.mojo</groupId>
+        <artifactId>build-helper-maven-plugin</artifactId>
+        <executions>
+          <execution>
+            <goals>
+              <goal>add-source</goal>
+            </goals>
+            <phase>generate-sources</phase>
+            <configuration>
+              <sources>
+                <source>src/3rdparty/java</source>
+              </sources>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
+
 </project>