Ignore the generated_classes created by the JUnit tests

These are written out when debugging tests from within Eclipse.

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/.gitignore b/.gitignore
index 903c6c8..d145da5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 /target
+/generated_classes
 /.classpath
 /.project
 /.settings/org.maven.ide.eclipse.prefs