tools/gdbsx: drop stray recursion into tools/include/

tools/gdbsx: drop stray recursion into tools/include/

Doing so isn't appropriate here - this gets done very early in the build
process. If the directory is mean to to be buildable on its own,
different arrangements would be needed.

The issue has become more pronounced by 47654a0d7320 ("tools/include:
fix (drop) dependencies of when to populate xen/"), but was there before
afaict.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>