xen: add (EXPERT) to one-line description of XEN_SHSTK

xen: add (EXPERT) to one-line description of XEN_SHSTK

Add an "(EXPERT)" tag to the one-line description of Kconfig options
that depend on EXPERT. (Not where just the prompt depends on EXPERT.)

Today we only have one such option: XEN_SHSTK.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
CC: andrew.cooper3@citrix.com
CC: george.dunlap@citrix.com
CC: iwj@xenproject.org
CC: jbeulich@suse.com
CC: julien@xen.org
CC: wl@xen.org