refactor: Use TranslateFn type consistently

The type was introduced in the previous commit.

src/kernel/bitcoinkernel.cpp

@@ -1,10 +1,11 @@
 // Copyright (c) 2022 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#include <util/translation.h>
 
 #include <functional>
 #include <string>
 
 // Define G_TRANSLATION_FUN symbol in libbitcoinkernel library so users of the
 // library aren't required to export this symbol
-extern const std::function<std::string(const char*)> G_TRANSLATION_FUN = nullptr;
+extern const TranslateFn G_TRANSLATION_FUN{nullptr};