Make UnPivot.getUnPivotInClause() return List<SelectExpressionItem> (#1039)

Author: moonfruit

src/main/java/net/sf/jsqlparser/statement/select/UnPivot.java

@@ -54,7 +54,7 @@ public void setUnPivotForClause(List<Column> forColumns) {
         this.unpivotForClause = forColumns;
     }
 
-    public List<?> getUnPivotInClause() {
+    public List<SelectExpressionItem> getUnPivotInClause() {
         return unpivotInClause;
     }