Refactor out translation.h

This is a prerequisite for introducing bilingual error messages.
Note: #includes are arranged by clang-format-diff.py script.

src/bitcoind.cpp

@@ -19,6 +19,9 @@
 #include <util/strencodings.h>
 #include <util/system.h>
 #include <util/threadnames.h>
+#include <util/translation.h>
+
+#include <functional>
 
 const std::function<std::string(const char*)> G_TRANSLATION_FUN = nullptr;