compiler warnings: use enum ranap_nsap_addr_enc, constify local var

Use the proper enum ranap_nsap_addr_enc instead of int, and properly exclude
that member when we're building without Iu support:

sgsn_vty.c:1323:31: error: passing argument 2 of ‘ranap_iu_vty_init’ from incompatible pointer type [-Werror=incompatible-pointer-types]
  ranap_iu_vty_init(SGSN_NODE, &g_cfg->iu.rab_assign_addr_enc);

Add const to a local var to silence compiler warning retrieving TLVP_VAL:

gprs_gmm.c:1657:18: error: initialization discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
    uint8_t *mi = TLVP_VAL(&tp, GSM48_IE_GMM_ALLOC_PTMSI);
                  ^~~~~~~~

Change-Id: I1168ce6425c31db3f6c3bf1f3682ae96b028c59b
diff --git a/src/gprs/gprs_gmm.c b/src/gprs/gprs_gmm.c
index c9fb8da..ff993cc 100644
--- a/src/gprs/gprs_gmm.c
+++ b/src/gprs/gprs_gmm.c
@@ -1654,7 +1654,7 @@
 			/* In Iu mode search only for ptmsi */
 			char mi_string[GSM48_MI_SIZE];
 			uint8_t mi_len = TLVP_LEN(&tp, GSM48_IE_GMM_ALLOC_PTMSI);
-			uint8_t *mi = TLVP_VAL(&tp, GSM48_IE_GMM_ALLOC_PTMSI);
+			const uint8_t *mi = TLVP_VAL(&tp, GSM48_IE_GMM_ALLOC_PTMSI);
 			uint8_t mi_type = *mi & GSM_MI_TYPE_MASK;
 			uint32_t tmsi;