PathServlet: Suppress ImmutableEnumChecker warning

FileMode is effectively immutable, and it's a JGit class that we
don't control here, so just suppress the warning.

Change-Id: Ia039bdf52569aae76f9c11cd0fab01bd997175f8

java/com/google/gitiles/PathServlet.java

diff --git a/java/com/google/gitiles/PathServlet.java b/java/com/google/gitiles/PathServlet.java
index 34e0a76..df24062 100644
--- a/java/com/google/gitiles/PathServlet.java
+++ b/java/com/google/gitiles/PathServlet.java
@@ -95,6 +95,7 @@
     EXECUTABLE_FILE(FileMode.EXECUTABLE_FILE),
     GITLINK(FileMode.GITLINK);
 
+    @SuppressWarnings("ImmutableEnumChecker") // FileMode is effectively immutable.
     private final FileMode mode;
 
     FileType(FileMode mode) {