Use OSMO_FD_* instead of deprecated BSC_FD_*

New define is available since libosmocore 1.1.0, and we already require
1.1.0, so no need to update dependenices.
Let's change it to avoid people re-using old BSC_FD_* symbols when
copy-pasting somewhere else.

Change-Id: I9b6463af713f76c06a144bdbf202c0d91eef4d21
diff --git a/src/libosmo-mgcp/mgcp_osmux.c b/src/libosmo-mgcp/mgcp_osmux.c
index 72d3b91..a1121de 100644
--- a/src/libosmo-mgcp/mgcp_osmux.c
+++ b/src/libosmo-mgcp/mgcp_osmux.c
@@ -406,7 +406,7 @@
 		return ret;
 	}
 	mgcp_set_ip_tos(osmux_fd.fd, cfg->endp_dscp);
-	osmux_fd.when |= BSC_FD_READ;
+	osmux_fd.when |= OSMO_FD_READ;
 
 	ret = osmo_fd_register(&osmux_fd);
 	if (ret < 0) {