Use OSMO_FD_* instead of deprecated BSC_FD_*

New define is available since libosmocore 1.1.0, and we already require
1.3.0, 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: Ifc89fffac0443d94f3e49555684975b293ef90fb
diff --git a/src/libmsc/smpp_smsc.c b/src/libmsc/smpp_smsc.c
index df3c847..2d82d98 100644
--- a/src/libmsc/smpp_smsc.c
+++ b/src/libmsc/smpp_smsc.c
@@ -931,7 +931,7 @@
 	osmo_wqueue_init(&esme->wqueue, 10);
 	esme->wqueue.bfd.fd = fd;
 	esme->wqueue.bfd.data = esme;
-	esme->wqueue.bfd.when = BSC_FD_READ;
+	esme->wqueue.bfd.when = OSMO_FD_READ;
 
 	if (osmo_fd_register(&esme->wqueue.bfd) != 0) {
 		close(fd);