update tus_chksum_update() stub

src/tus_blob_stub.c

@@ -103,7 +103,7 @@ tus_chksum_hdr(VRT_CTX, const char *hdr)
 }
 
 void
-tus_chksum_update(VRT_CTX, struct tus_chksum *c,
+tus_chksum_update(VRT_CTX, const struct tus_chksum *c,
     const void *ptr, size_t l)
 {
 	(void) ctx;