soft_uart: add osmo_soft_uart_free()

Change-Id: I2fdcf6116144d8f16cf4167c37cfa7215d16337f
Related: OS#4396
diff --git a/src/core/libosmocore.map b/src/core/libosmocore.map
index c08d3ff..142e4c4 100644
--- a/src/core/libosmocore.map
+++ b/src/core/libosmocore.map
@@ -440,6 +440,7 @@
 osmo_sock_unix_init;
 osmo_sock_unix_init_ofd;
 osmo_soft_uart_alloc;
+osmo_soft_uart_free;
 osmo_soft_uart_configure;
 osmo_soft_uart_enable;
 osmo_soft_uart_rx_ubits;