soft_uart: allow manually flushing the receive buffer

Change-Id: Id600a2db99e6cb84866cbdcfcd4f78265e067291
Related: OS#4396
diff --git a/src/core/libosmocore.map b/src/core/libosmocore.map
index e6a1866..ae8a068 100644
--- a/src/core/libosmocore.map
+++ b/src/core/libosmocore.map
@@ -448,6 +448,7 @@
 osmo_soft_uart_rx_ubits;
 osmo_soft_uart_tx_ubits;
 osmo_soft_uart_set_status;
+osmo_soft_uart_flush_rx;
 osmo_stat_item_dec;
 osmo_stat_item_flush;
 osmo_stat_item_for_each_group;