fix an error revealed by compiling with clang

src/vmod_blobdigest.c

@@ -158,7 +158,7 @@ final(const enum algorithm hash, hash_ctx *restrict const hctx,
 
 static inline void
 digest(const enum algorithm hash, hash_ctx *restrict const hctx,
-       const VCL_BLOB restrict const b, uint8_t *restrict digest)
+       VCL_BLOB restrict const b, uint8_t *restrict digest)
 {
 	init(hash, hctx);
 	update(hash, hctx, b->priv, b->len);