xen: Add stdbool.h workaround for BSD.

xen: Add stdbool.h workaround for BSD.

On *BSD, stdbool.h lives in /usr/include, but we don't want to have
that on the search path in case we pick up any headers from the build
host's C libraries.

Copy the equivalent hack already in place for stdarg.h: on all
supported compilers the contents of stdbool.h are trivial, so just
supply the things we need in a xen/stdbool.h header.

Signed-off-by: Tim Deegan <tim@xen.org>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
Tested-by: Patrick Welche <prlw1@cam.ac.uk>