fix compiler warning: msg_utils.c: fn_chk() constify arg
fix compiler warning: msg_utils.c: fn_chk() constify arg

The only caller dtx_sched_optional() passes const arrays and a warning was
issued.

Change-Id: I18e0788ef96489d2116a60cae57bd516fe6eae8e

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts