Fix Maven missing version warning

[WARNING] Some problems were encountered while building the effective
model for org.eclipse.jgit:org.eclipse.jgit.benchmarks:jar:5.6.0-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for
org.apache.maven.plugins:maven-shade-plugin is missing. @ line 131,
column 15

Change-Id: Id25916c460bc4a10d0f797779d24f53292df2b77
Signed-off-by: Michael Keppler <Michael.Keppler@gmx.de>

org.eclipse.jgit.benchmarks/pom.xml

@@ -131,6 +131,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-shade-plugin</artifactId>
+        <version>3.2.1</version>
         <executions>
           <execution>
             <phase>package</phase>