gsm_08_58: add flexible array member to 'struct ipac_preproc_ave_cfg'

Some averaging methods may have additional parameters, so let's
make it easier to access them for the API user.

Change-Id: I2f4ed56837dd479dbbd10c0a7df0ed7565d3946a
Related: SYS#4918
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 33e27af..e8fb32b 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -9,5 +9,6 @@
 #library	what			description / commit summary line
 libosmogsm	new API			gsm0808_create_sapi_reject_cause() with cause argument
 libosmogsm	ABI change		struct osmo_gprs_cell_options size changed (new field at the end)
+libosmogsm	API change		struct ipac_preproc_ave_cfg: new zero-sized flexible array member at end
 libosmovty	ABI change		struct cmd_element: add a field for program specific attributes
 libosmovty	ABI change		struct vty_app_info: optional program specific attributes description