MAINTAINERS: add files related to kdump

Kdump is implemented based on kexec, however some files are only related
to crash dumping and missing, add them to KDUMP entry.

Signed-off-by: Baoquan He <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Acked-by: Dave Young <[e-mail]>
Link: http://lkml.kernel.org/r/20200520103633.GW5029@MiWiFi-R3L-srv
Signed-off-by: Linus Torvalds <[e-mail]>

MAINTAINERS

···
 S:	Maintained
 W:	http://lse.sourceforge.net/kdump/
 F:	Documentation/admin-guide/kdump/
+F:	fs/proc/vmcore.c
+F:	include/linux/crash_core.h
+F:	include/linux/crash_dump.h
+F:	include/uapi/linux/vmcore.h
+F:	kernel/crash_*.c
 
 KEENE FM RADIO TRANSMITTER DRIVER
 M:	Hans Verkuil <[e-mail]>