Revert incorrect removal of the XDR currency from locale/iso-4217.def

In 7447ccd9, the XDR currency was
removed from locale/iso-4217.def, despite the fact that it's both
still a part of the standard, according to the official table:

http://www.currency-iso.org/dam/downloads/table_a1.xml

... and, more importantly, is referenced from localedata/i18n, so
any quick-and-dirty locale definition that uses "copy i18n" for
LC_MONETARY wouldn't work anymore.