libnm: add required-timeout backported symbol from 1.30.8

The nm_setting_ip_config_get_required_timeout() symbol was introduced
in libnm 1.32.4 and then backported to 1.30.8.

Export it also with version @libnm_1_30_8; this allows a program built
against libnm 1.30.8 to keep working with later versions of the
library.

src/libnm-client-impl/libnm.ver

@@ -1785,6 +1785,10 @@ global:
 	nm_utils_print;
 } libnm_1_28_0;
 
+libnm_1_30_8 {
+	#nm_setting_ip_config_get_required_timeout@libnm_1_30_8;
+} libnm_1_30_0;
+
 libnm_1_32_0 {
 global:
 	nm_ethtool_optname_is_pause;

src/libnm-client-impl/nm-client.c

@@ -8832,3 +8832,9 @@ NM_BACKPORT_SYMBOL(libnm_1_10_14,
                    (NMSettingConnection * setting),
                    (setting));
 NM_BACKPORT_SYMBOL(libnm_1_10_14, GType, nm_setting_connection_mdns_get_type, (void), ());
+
+NM_BACKPORT_SYMBOL(libnm_1_30_8,
+                   int,
+                   nm_setting_ip_config_get_required_timeout,
+                   (NMSettingIPConfig * setting),
+                   (setting));