gsup: Decode/Encode the hlr-Number in the GSUP message

Implement it similar to the msisdn_enc/msisdn_enc_len and
extend the testcase to include it as well.
diff --git a/openbsc/tests/gprs/gprs_test.c b/openbsc/tests/gprs/gprs_test.c
index bbd1d8a..c78b98a 100644
--- a/openbsc/tests/gprs/gprs_test.c
+++ b/openbsc/tests/gprs/gprs_test.c
@@ -445,6 +445,8 @@
 		TEST_IMSI_IE,
 		0x08, 0x07, /* MSISDN of the subscriber */
 			0x91, 0x94, 0x61, 0x46, 0x32, 0x24, 0x43,
+		0x09, 0x07, /* HLR-Number of the subscriber */
+			0x91, 0x83, 0x52, 0x38, 0x48, 0x83, 0x93,
 		0x04, 0x00, /* PDP info complete */
 		0x05, 0x15,
 			0x10, 0x01, 0x01,