Use Bazel Preconditions variant which avoids varargs array creation

Reduces garbage.

--
MOS_MIGRATED_REVID=109914243
diff --git a/src/main/java/com/google/devtools/build/lib/vfs/UnionFileSystem.java b/src/main/java/com/google/devtools/build/lib/vfs/UnionFileSystem.java
index a1ef71f..28cf7df 100644
--- a/src/main/java/com/google/devtools/build/lib/vfs/UnionFileSystem.java
+++ b/src/main/java/com/google/devtools/build/lib/vfs/UnionFileSystem.java
@@ -14,9 +14,9 @@
 
 package com.google.devtools.build.lib.vfs;
 
-import com.google.common.base.Preconditions;
 import com.google.common.collect.Lists;
 import com.google.devtools.build.lib.concurrent.ThreadSafety;
+import com.google.devtools.build.lib.util.Preconditions;
 import com.google.devtools.build.lib.util.StringTrie;
 
 import java.io.IOException;