BACKPORT: lib: zstd: Fix unused variable warning

Backport the fix from upstream PR #2838 [0]. Found by the Kernel test
robot in [1].

[0] facebook/zstd#2838
[1] https://lore.kernel.org/linux-mm/202111120312.833wII4i-lkp@intel.com/T/

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Nick Terrell <terrelln@fb.com>
[cyberknight777: backport to 4.14]
Signed-off-by: Cyber Knight <cyberknight755@gmail.com>

lib/zstd/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;