ia64: fix make install under tools/debugger/xenitp

ia64: fix make install under tools/debugger/xenitp

This patch fixes the following error with make install under
the directory, tools/debugger/xenitp by checking whether
the variable is length zero string.

> # make install
> ../../../tools/cross-install -d -m0755 -p //usr/bin
> [ -z " xenitp" ] || ../../../tools/cross-install -d -m0755 -p
> //usr/lib/xen/bin
> ../../../tools/cross-install -d -m0755 -p //usr/share/man/man1
> ../../../tools/cross-install -d -m0755 -p //usr/share/man/man8
> ../../../tools/cross-install -m0755 -p   //usr/bin
> install: too few arguments
> Try `install --help' for more information.
> make: *** [install] Error 1

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>