osmo_bsc_bssap.c: Fix discard of const qualifier in assignment

Fixes following compilation warning:
osmo-bsc/src/osmo-bsc/osmo_bsc_bssap.c:442:10: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  enc_key = &data[1];
          ^

Change-Id: Ieeab5a822d50ac1267362f57196b80073a7901f9
diff --git a/src/osmo-bsc/osmo_bsc_bssap.c b/src/osmo-bsc/osmo_bsc_bssap.c
index 45861cc..0ecc11c 100644
--- a/src/osmo-bsc/osmo_bsc_bssap.c
+++ b/src/osmo-bsc/osmo_bsc_bssap.c
@@ -401,7 +401,7 @@
 	struct msgb *resp;
 	int reject_cause = -1;
 	int include_imeisv = 1;
-	uint8_t *enc_key;
+	const uint8_t *enc_key;
 	uint16_t enc_key_len;
 	uint8_t enc_bits_bsc;
 	uint8_t enc_bits_msc;