Refactor out translation.h

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

src/validation.cpp

@@ -41,6 +41,7 @@
 #include <util/rbf.h>
 #include <util/strencodings.h>
 #include <util/system.h>
+#include <util/translation.h>
 #include <util/validation.h>
 #include <validationinterface.h>
 #include <warnings.h>