Fix build of mail/xpbiff after libXt update.

An implicit include has gone away (Intrinsic.h) so users of X11/StringDefs.h
now need to explictly include X11/Intrinsic.h to get the typedef for
String, otherwise many of the macro's like this one:

#define XtRString ((String)&XtStrings[1797])

will fall over.