stubdom: allow to build with older tool chain

stubdom: allow to build with older tool chain

GNU make prior to 3.81 doesn't support $(realpath ...). This fixes a
regression introduced in 23368:0f670f5146c8 (the option tested via
cc-option-add got interpreted as the argument of the -I compiler
option, as its intended argument was blank, and hence the compiler was
falsely considered to support *any* option in the pciutils sub-tree).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>