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 6c122d3984 ("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>

xen/Makefile

@@ -150,10 +150,10 @@ config: FORCE
 else # !config-build
 
 ifeq ($(need-config),y)
-include include/config/auto.conf
+-include include/config/auto.conf
 # Read in dependencies to all Kconfig* files, make sure to run syncconfig if
 # changes are detected.
-include include/config/auto.conf.cmd
+-include include/config/auto.conf.cmd
 
 # Allow people to just run `make` as before and not force them to configure
 $(KCONFIG_CONFIG):