Fix unused variable warning

`litLengthSum` is unused when asserts are disabled. Already fixed in `dev` by
PR#2838. Found by the Kernel test robot [0].

[0] https://lore.kernel.org/linux-mm/202111120312.833wII4i-lkp@intel.com/T/

Reported-by: kernel test robot <lkp@intel.com>

lib/compress/zstd_compress_superblock.c

@@ -411,6 +411,7 @@ static size_t ZSTD_seqDecompressedSize(seqStore_t const* seqStore, const seqDef*
     const seqDef* sp = sstart;
     size_t matchLengthSum = 0;
     size_t litLengthSum = 0;
+    (void)litLengthSum;
     while (send-sp > 0) {
         ZSTD_sequenceLength const seqLen = ZSTD_getSequenceLength(seqStore, sp);
         litLengthSum += seqLen.litLength;