SoyConstants: Add private default constructor

Change-Id: Ifc8e437faa9d4a7327cf2f9d07e0e5547a3adf45

java/com/google/gitiles/doc/SoyConstants.java

diff --git a/java/com/google/gitiles/doc/SoyConstants.java b/java/com/google/gitiles/doc/SoyConstants.java
index a69cc6a..48f1ede 100644
--- a/java/com/google/gitiles/doc/SoyConstants.java
+++ b/java/com/google/gitiles/doc/SoyConstants.java
@@ -47,4 +47,6 @@
    * com.google.template.soy.shared.internal.EscapingConventions.FilterNormalizeUri}.
    */
   static String NORMAL_URI_INNOCUOUS_OUTPUT = "about:invalid#zSoyz";
+
+  private SoyConstants() {}
 }