b-cs: Define G_TRANSLATION_FUN in bitcoinkernel.cpp

[META] This is done in preparation for extracting libbitcoinkernel in a
       following commit. It seems logical that generally users of a
       library shouldn't need to export its own symbols to use the
       library.

src/kernel/bitcoinkernel.cpp

@@ -0,0 +1,10 @@
+// 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 <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;