ltq-vdsl-vr11-mei: fix missing-prototypes build warnings

ltq-vdsl-vr11-mei: fix missing-prototypes build warnings

Add prototype definition for exported function ifx_mei_atm_led_blink().

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/18744
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit eb18b602da639577573930fd4e800ed716928b4f)
Signed-off-by: Nora Matthias Schiffer <neocturne@universe-factory.net>
Link: https://github.com/openwrt/openwrt/pull/20440
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>