MAINTAINERS: add kexec_core.c and kexec_file.c

In the below commits kexec.c was split to kexec.c, kexec_file.c and
kexec_core.c.

commit a43cac0d ("kexec: split kexec_file syscall code to kexec_file.c")
commit 2965faa5

 ("kexec: split kexec_load syscall from kexec core code")

Both kexec_file.c and kexec_core.c still belong to the kexec component.
In order to get correct mail lists by using the script get_maintainer.pl,
add these files to MAINTAINERS.

Link: http://lkml.kernel.org/r/1464189735-59113-1-git-send-email-mnghuan@gmail.com
Signed-off-by: Minfei Huang <mnghuan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>