[SPARK-18634][SQL][TRIVIAL] Touch-up Generate

## What changes were proposed in this pull request?
I jumped the gun on merging https://github.com/apache/spark/pull/16120, and missed a tiny potential problem. This PR fixes that by changing a val into a def; this should prevent potential serialization/initialization weirdness from happening.

## How was this patch tested?
Existing tests.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #16170 from hvanhovell/SPARK-18634.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala

@@ -93,7 +93,7 @@ case class Generate(
 
   override def producedAttributes: AttributeSet = AttributeSet(generatorOutput)
 
-  val qualifiedGeneratorOutput: Seq[Attribute] = qualifier.map { q =>
+  def qualifiedGeneratorOutput: Seq[Attribute] = qualifier.map { q =>
     // prepend the new qualifier to the existed one
     generatorOutput.map(a => a.withQualifier(Some(q)))
   }.getOrElse(generatorOutput)