modinst: wrap long lines in order to enhance cmd_modules_install

Note: shouldn't we use 'install -D $(2)/$@ $@' instead of mkdir
and cp ?

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