Include |SSL_CTX_set_signing_algorithm_prefs| in SSL_CTX fuzzer.

I missed this function, which was unfortunate.

Change-Id: I8bcea1738a50aa3297d09a59a86437351ff5f84a
Reviewed-on: https://boringssl-review.googlesource.com/11623
Commit-Queue: Adam Langley <alangley@gmail.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>

fuzz/ssl_ctx_api.cc

diff --git a/fuzz/ssl_ctx_api.cc b/fuzz/ssl_ctx_api.cc
index 4388065..4af80c2 100644
--- a/fuzz/ssl_ctx_api.cc
+++ b/fuzz/ssl_ctx_api.cc
@@ -330,6 +330,16 @@
           ocsp_data.size());
     },
     [](SSL_CTX *ctx, CBS *cbs) {
+      std::string signing_algos;
+      if (!GetString(&signing_algos, cbs)) {
+        return;
+      }
+
+      SSL_CTX_set_signing_algorithm_prefs(
+          ctx, reinterpret_cast<const uint16_t *>(signing_algos.data()),
+          signing_algos.size() / sizeof(uint16_t));
+    },
+    [](SSL_CTX *ctx, CBS *cbs) {
       std::string ciphers;
       if (!GetString(&ciphers, cbs)) {
         return;