Add @SuppressWarnings("serial") to an Exception.

Test Plan: Eliminates a warning in Eclipse.
diff --git a/src/com/facebook/buck/httpserver/WebServer.java b/src/com/facebook/buck/httpserver/WebServer.java
index 720e649..eb34193 100644
--- a/src/com/facebook/buck/httpserver/WebServer.java
+++ b/src/com/facebook/buck/httpserver/WebServer.java
@@ -86,6 +86,7 @@
     }
   }
 
+  @SuppressWarnings("serial")
   public class WebServerException extends Exception {
 
     public WebServerException(String message, Exception clause) {