Declare tls::set_verify_callback() for the split test build

The ClientCertStillRequired tests call tls::set_verify_callback(), whose
declaration lives below the split border, so test_split failed to compile.
Declare it next to create_ca_store(), which the split build needs for the
same reason.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WGAYpQgXt95LdDQcJYgjB8

test/test.cc

@@ -69,6 +69,7 @@ namespace tls {
 // Declared here for the split build, where the TLS abstraction declarations
 // live below the split border; the definition is linked from httplib.cc.
 ca_store_t create_ca_store(const char *pem, size_t len);
+bool set_verify_callback(ctx_t ctx, VerifyCallback callback);
 } // namespace tls
 } // namespace httplib
 #endif