debug/gdb: always enable expat for the cross-gdb

There's no point in not supporting XML in the cross-gdb.
I mean, come on... ;-)

It's still the responsibility of the user to have the necessary
devel expat packages installed for his/her distro.

Reported-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 This is NOT the official version of dialog.  This version has been

 significantly modified from the original.  It is for use by the Linux

 kernel configuration script.  Please do not bother Savio Lam with 

 questions about this program.