force inclusion of xen/config.h through compiler option

force inclusion of xen/config.h through compiler option

As we expect all source files to include the header as the first thing
anyway, stop doing this by repeating the inclusion in each and every
source file (and in many headers), but rather enforce this uniformly
through the compiler command line.

As a first cleanup step, remove the explicit inclusion from all common
headers. Further cleanup can be done incrementally.

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