irq-gic: remove file name from heading comment

File names in the heading comments  fell out of favor long ago, and this one
weren't even changed when the driver was moved from arch/arm/common/, so remove
it at last...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>