ltq-ifxos: fix missing-prototypes build warning

ltq-ifxos: fix missing-prototypes build warning

Add prototype definition for exported function IFX_Var_Fifo_getRoom().

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 e9d3e0f1dacc8a8052db3e45562ca6c71a496f09)
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>