Add missing @Override to DroidDoc and ApiCheck

Change-Id: Ic7829a4ea62a614ef8b525bd84f8fbaaa4674d06
diff --git a/tools/droiddoc/src/Errors.java b/tools/droiddoc/src/Errors.java
index 95439f1..77852f8 100644
--- a/tools/droiddoc/src/Errors.java
+++ b/tools/droiddoc/src/Errors.java
@@ -41,6 +41,7 @@
             return this.msg.compareTo(that.msg);
         }
 
+        @Override
         public String toString() {
             String whereText = this.pos == null ? "unknown: " : this.pos.toString() + ':';
             return whereText + this.msg;