CLEANUP: ssl: make ssl_sock_generated_cert_serial() take a const

It doesn't modify the input.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index aa7bb1b..aa1d2ae 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -1145,7 +1145,7 @@
 
 /* Compute the serial that will be used to create/set/get a certificate. */
 unsigned int
-ssl_sock_generated_cert_serial(void *data, size_t len)
+ssl_sock_generated_cert_serial(const void *data, size_t len)
 {
 	return XXH32(data, len, ssl_ctx_lru_seed);
 }