xterm: x-includes and x-libraries must be set for cross-compiling

These flags need to be set so that the configure script would
correctly use libICE from cross-toolchain rather than from host.

This fix is similar to "package/efl/libevas: x-includes and x-libraries
must be set for cross-compiling" done by Romain Naour on libecore.

Signed-off-by: Mikhail Peselnik <bas@bmail.ru>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>