osmo-bts-trx: cosmetic: use '#pragma once' in trx_if.h
osmo-bts-trx: cosmetic: use '#pragma once' in trx_if.h

Change-Id: I78b93a8f5fbbd94141d30518d37c1ce05f8a681f

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