JschConfigSessionFactory: [findbugs] Set synchronized

Set synchronized to make the config access consistent.

> Inconsistent synchronization of
org.eclipse.jgit.transport.JschConfigSessionFactory.config; locked 80%
of time

In order to make JschConfigSessionFactory threadsafe, synchronize this
method as well.

Change-Id: I32d1bfc2e98363d254992144e795ce72fe1e8846
Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/transport/JschConfigSessionFactory.java

@@ -551,7 +551,7 @@ public abstract class JschConfigSessionFactory extends SshSessionFactory {
 	 * @param config
 	 *            to use
 	 */
-	void setConfig(OpenSshConfig config) {
+	synchronized void setConfig(OpenSshConfig config) {
 		this.config = config;
 	}
 }