tools/xenstored: xenstored_core.h should include fcntl.h

tools/xenstored: xenstored_core.h should include fcntl.h

xenstored_core.h will consider live-udpate is not supported if
O_CLOEXEC doesn't exist. However, the header doesn't include the one
defining O_CLOEXEC (i.e. fcntl.h). This means that depending on
the header included, some source file will think Live-Update is not
supported.

I am not aware of any issue with the existing. Therefore this is just
a latent bug so far.

Prevent any potential issue by including fcntl.h in xenstored_core.h

Fixes: cd831ee438 ("tools/xenstore: handle CLOEXEC flag for local files and pipes")
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Juergen Gross <jgross@suse.com>