Fix validate for GHCi without TABLES_NEXT_TO_CODE

Suppress warning about unused match.

Fixes #15187

Reviewers: bgamari, simonmar, erikd, hvr

Reviewed By: bgamari, simonmar

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4741

libraries/ghci/GHCi/InfoTable.hsc

@@ -320,11 +320,11 @@ data StgConInfoTable = StgConInfoTable {
 pokeConItbl
   :: Ptr StgConInfoTable -> Ptr StgConInfoTable -> StgConInfoTable
   -> IO ()
-pokeConItbl wr_ptr ex_ptr itbl = do
+pokeConItbl wr_ptr _ex_ptr itbl = do
 #if defined(TABLES_NEXT_TO_CODE)
   -- Write the offset to the con_desc from the end of the standard InfoTable
   -- at the first byte.
-  let con_desc_offset = conDesc itbl `minusPtr` (ex_ptr `plusPtr` conInfoTableSizeB)
+  let con_desc_offset = conDesc itbl `minusPtr` (_ex_ptr `plusPtr` conInfoTableSizeB)
   (#poke StgConInfoTable, con_desc) wr_ptr con_desc_offset
 #else
   -- Write the con_desc address after the end of the info table.