Fix a -Wdeprecated-copy warning.

Bug: chromium:1221591
Change-Id: Ie8335e53b107ba019a1bde62c12f846802e056c5
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/48165
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

include/openssl/bytestring.h

@@ -51,6 +51,7 @@ struct cbs_st {
   // Defining any constructors requires we explicitly default the others.
   cbs_st() = default;
   cbs_st(const cbs_st &) = default;
+  cbs_st &operator=(const cbs_st &) = default;
 #endif
 };