fix "-Werror" build against latest libosmo-abis

libosmo-abisrecently marked the 'out_cb' of the subchan_demux
as 'const', which caused compiler warnings/errors.

Related: libosmo-abis.git Ia082b9fddf03d02afd007825a1588a3ef0dbedae
Change-Id: I0cf430980e50fa8094f0efc7642004fb5923c4c6
3 files changed