Fix CStruct annotation processor warning.

Updated the Java source version the annotation processor supports
to stop it from throwing a warning during the build process.

src/java/com/jogamp/gluegen/structgen/CStructAnnotationProcessor.java

@@ -78,7 +78,7 @@
  * @author Sven Gothel, et.al.
  */
 @SupportedAnnotationTypes(value = {"com.jogamp.gluegen.structgen.CStruct"})
-@SupportedSourceVersion(SourceVersion.RELEASE_6)
+@SupportedSourceVersion(SourceVersion.RELEASE_7)
 public class CStructAnnotationProcessor extends AbstractProcessor {
     private static final String DEFAULT = "_default_";
     private static final boolean DEBUG;