Ignore warnings for generated source code in org.eclipse.jgit.benchmark

The source code in the folder .apt_generated is generated by the JMH
code generator, so there's no point in raising any warnings as this
could only be fixed in the upstream code generator.

Change-Id: I882888e7bf924f9ae74182598fcb91671a5c9818
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.benchmarks/.classpath

@@ -18,6 +18,7 @@
 	</classpathentry>
 	<classpathentry kind="src" path=".apt_generated">
 		<attributes>
+			<attribute name="ignore_optional_problems" value="true"/>
 			<attribute name="optional" value="true"/>
 			<attribute name="maven.pomderived" value="true"/>
 			<attribute name="ignore_optional_problems" value="true"/>