Fix some warnings when compiling under MSVC.

FossilOrigin-Name: 708338773919fa024abbba55180654beb60e6d84

src/os_win.c

@@ -1657,6 +1657,7 @@ static int winShmLock(
 static void winShmBarrier(
   sqlite3_file *fd          /* Database holding the shared memory */
 ){
+  UNUSED_PARAMETER(fd);
   /* MemoryBarrier(); // does not work -- do not know why not */
   winShmEnterMutex();
   winShmLeaveMutex();