Don't modify WindowSpec.

Author: viirya

sql/core/src/main/scala/org/apache/spark/sql/expressions/WindowSpec.scala

@@ -18,6 +18,7 @@
 package org.apache.spark.sql.expressions
 
 import org.apache.spark.annotation.Experimental
+import org.apache.spark.sql.types.BooleanType
 import org.apache.spark.sql.{Column, catalyst}
 import org.apache.spark.sql.catalyst.expressions._