xen/build: silence make warnings about missing auto.conf*

xen/build: silence make warnings about missing auto.conf*

In a clean tree, both files include/config/auto.conf{,.cmd} are
missing and older version of GNU Make complain about it:
    Makefile:103: include/config/auto.conf: No such file or directory
    Makefile:106: include/config/auto.conf.cmd: No such file or directory

Those warnings are harmless, make will create the files and start over. But
to avoid confusion, we'll use "-include" to silence the warning.

Those warning started to appear with commit 6c122d3984a5 ("xen/build:
include include/config/auto.conf in main Makefile").

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>