MAINTAINERS: Add IRQ subsystem

I do not expect, that this will help anything, but at least it's going
to remove the lame excuse about the missing maintainer entry.

Signed-off-by: Thomas Gleixner <[e-mail]>

MAINTAINERS

···
 F:	include/net/irda/
 F:	net/irda/
 
+IRQ SUBSYSTEM
+M:	Thomas Gleixner <[e-mail]>
+S:	Maintained
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip.git irq/core
+F:	kernel/irq/
+
 ISAPNP
 M:	Jaroslav Kysela <[e-mail]>
 S:	Maintained