Add comment to unused variable suppression

lib/compress/zstd_compress_superblock.c

diff --git a/lib/compress/zstd_compress_superblock.c b/lib/compress/zstd_compress_superblock.c
index 6d44add..9c5fb06 100644
--- a/lib/compress/zstd_compress_superblock.c
+++ b/lib/compress/zstd_compress_superblock.c
@@ -411,6 +411,7 @@
     const seqDef* sp = sstart;
     size_t matchLengthSum = 0;
     size_t litLengthSum = 0;
+    /* Only used by assert(), suppress unused variable warnings in production. */
     (void)litLengthSum;
     while (send-sp > 0) {
         ZSTD_sequenceLength const seqLen = ZSTD_getSequenceLength(seqStore, sp);