LibUnicode: Add some data related to currency codes

This data is published under ISO-4217 as an XML file. Since we can't
parse XML files yet, and the data isn't very large, it was translated to
C++ manually here.

Userland/Libraries/LibUnicode/Forward.h

@@ -21,6 +21,7 @@ enum class Script : u8;
 enum class Territory : u8;
 enum class WordBreakProperty : u8;
 
+struct CurrencyCode;
 struct Keyword;
 struct LanguageID;
 struct ListPatterns;