Kexec: Update URL in MAINTAINERS file

I'm not sure that the new URL satifies the requirement of status/info, but
it does at least as good a job as the old URL, and contains current
releases of kexec-tools, rather than somewhat ancient versions.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>