modsign: lookup lines ending in .ko in .mod files

This does the same as commit ef591a55 (scripts/Makefile.modpost: error
in finding modules from .mod files), but for scripts/Makefile.modsign

Maybe we should also apply to Makefile.modsign and Makefile.modinst
the change applied to Makefile.modpost by commit ea4054a2 (modpost:
handle huge numbers of modules) ?

Reviewed-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Bertrand Jacquin <beber@meleeweb.net>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>