Fix HookCallingExternalCatalog.dropPartitions

## What changes were proposed in this pull request?
`HookCallingExternalCatalog.dropPartitions` was accidentally broken during the last merge with Apache Spark. This fixes the build.

## How was this patch tested?
Existing tests.

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

Closes apache#134 from hvanhovell/HotFix-HookCallingExternalCatalog.

sql/core/src/main/scala/org/apache/spark/sql/internal/HookCallingExternalCatalog.scala

@@ -213,8 +213,9 @@ class HookCallingExternalCatalog(val delegate: ExternalCatalog, val hooks: Catal
       table: String,
       parts: Seq[TablePartitionSpec],
       ignoreIfNotExists: Boolean,
-      purge: Boolean): Unit = {
-    delegate.dropPartitions(db, table, parts, ignoreIfNotExists, purge)
+      purge: Boolean,
+      retainData: Boolean): Unit = {
+    delegate.dropPartitions(db, table, parts, ignoreIfNotExists, purge, retainData)
   }
 
   override def renamePartitions(