libmsc: msc_vty: Fix compilation warning

As the include file gsm_data.h is generic (does not depend on osmo-iuh0s
iu_client.h), rab_assign_addr_enc is declared as "int" instead of "enum ranap_nsap_addr_enc".

osmo-msc/src/libmsc/msc_vty.c: In function ‘msc_vty_init’:
osmo-msc/src/libmsc/msc_vty.c:212:30: warning: passing argument 2 of ‘ranap_iu_vty_init’ from incompatible pointer type [-Wincompatible-pointer-types]
  ranap_iu_vty_init(MSC_NODE, &msc_network->iu.rab_assign_addr_enc);
                              ^

Change-Id: I1b63ee350911bdf772a2324fff55035275a455c4
diff --git a/src/libmsc/msc_vty.c b/src/libmsc/msc_vty.c
index c1c9f6b..82608c6 100644
--- a/src/libmsc/msc_vty.c
+++ b/src/libmsc/msc_vty.c
@@ -209,6 +209,6 @@
 
 	mgcp_client_vty_init(msc_network, MSC_NODE, &msc_network->mgw.conf);
 #ifdef BUILD_IU
-	ranap_iu_vty_init(MSC_NODE, &msc_network->iu.rab_assign_addr_enc);
+	ranap_iu_vty_init(MSC_NODE, (enum ranap_nsap_addr_enc*)&msc_network->iu.rab_assign_addr_enc);
 #endif
 }