src: make CipherBase::kind_ const

The cipher kind doesn't change over the lifetime of the cipher so make
it const.

PR-URL: #14122
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: bnoordhuis

src/node_crypto.h

@@ -476,7 +476,7 @@ class CipherBase : public BaseObject {
  private:
   EVP_CIPHER_CTX ctx_; /* coverity[member_decl] */
   bool initialised_;
-  CipherKind kind_;
+  const CipherKind kind_;
   unsigned int auth_tag_len_;
   char auth_tag_[EVP_GCM_TLS_TAG_LEN];
 };