Enable -Wunused-variable in velox/PACKAGE

Summary:
This diff enables compilation warning flags for the directory in question. Further details are in [this workplace post](https://fb.workplace.com/permalink.php?story_fbid=pfbid02XaWNiCVk69r1ghfvDVpujB8Hr9Y61uDvNakxiZFa2jwiPHscVdEQwCBHrmWZSyMRl&id=100051201402394).

This is a low-risk diff. There are **no run-time effects** and the diff has already been observed to compile locally. **If the code compiles, it works; test errors are spurious.**

#build_targets_regex[fbcode//velox/.*]

#buildmore - Be thorough

ig-no-test - No need to run instagram tests; build phase is sufficient.

#buildsonlynotests - **This diff has no runtime effects.**

 - If you approve of this diff, please use the "Accept & Ship" button :-)

(1 file modified.)

Reviewed By: palmje

Differential Revision: D67329075

fbshipit-source-id: 5e79499de8c363b81f621dfcca23f24df614745c

velox/exec/tests/OutputBufferManagerTest.cpp

@@ -1335,10 +1335,8 @@ TEST_P(AllOutputBufferManagerTest, printOutputBufferStats) {
       1);
 
   const int numPages = numDestinations;
-  int totalNumRows = 0;
-  int totalBytes = 0;
   for (int pageId = 0; pageId < numPages; ++pageId) {
-    const auto pageBytes = enqueue(taskId, pageId, rowType_, vectorSize);
+    enqueue(taskId, pageId, rowType_, vectorSize);
     fetchOneAndAck(taskId, pageId, 0);
   }