[MINOR][BUILD] Adds spark-warehouse/ to .gitignore

## What changes were proposed in this pull request?

Adds spark-warehouse/ to `.gitignore`.

## How was this patch tested?

N/A

Author: Cheng Lian <lian@databricks.com>

Closes #12929 from liancheng/gitignore-spark-warehouse.

.gitignore

@@ -72,6 +72,7 @@ metastore/
 metastore_db/
 sql/hive-thriftserver/test_warehouses
 warehouse/
+spark-warehouse/
 
 # For R session data
 .RData