pgm: add missing optional dependency to org.tukaani:xz

Otherwize the jgit command line does not support the xz archive format.

Change-Id: I64897e658e4ed761614948b9d432f26e83034f15
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.pgm/pom.xml

@@ -142,6 +142,12 @@
       <artifactId>org.eclipse.jgit.lfs.server</artifactId>
       <version>${project.version}</version>
     </dependency>
+
+    <dependency>
+      <groupId>org.tukaani</groupId>
+      <artifactId>xz</artifactId>
+      <optional>true</optional>
+    </dependency>
   </dependencies>
 
   <build>