Enable checkstyle trailing spaces rule

Enable the checkstyle rule that detects unneeded trailing spaces
and fix the violations of the rule.

Change-Id: I65567c87142c5a257f0230a55bc4376e770d416f
diff --git a/src/main/java/net/onrc/onos/core/linkdiscovery/internal/EventHistoryTopologyLink.java b/src/main/java/net/onrc/onos/core/linkdiscovery/internal/EventHistoryTopologyLink.java
index 9f9bed9..f172fdd 100644
--- a/src/main/java/net/onrc/onos/core/linkdiscovery/internal/EventHistoryTopologyLink.java
+++ b/src/main/java/net/onrc/onos/core/linkdiscovery/internal/EventHistoryTopologyLink.java
@@ -12,7 +12,7 @@
  */
 public class EventHistoryTopologyLink {
     // The following fields are not stored as String to save memory
-    // They should be converted to appropriate human-readable strings by 
+    // They should be converted to appropriate human-readable strings by
     // the front end (e.g. in cli in Python)
     public long srcSwDpid;
     public long dstSwDpid;