fix spelling in 'value_string' arrays: existAnt -> existEnt

It could be that this spelling variant was originally used in the
specs., but now at least in 3GPP TS 44.018 they use 'existEnt'.

Change-Id: I847de910411f2edf7cc45b8c296b43e65fed5447
diff --git a/src/gsm/gsm0411_smr.c b/src/gsm/gsm0411_smr.c
index 02b4199..03cf005 100644
--- a/src/gsm/gsm0411_smr.c
+++ b/src/gsm/gsm0411_smr.c
@@ -131,7 +131,7 @@
 	{ GSM411_RP_CAUSE_INV_TRANS_REF, "Invalid Transaction Reference" },
 	{ GSM411_RP_CAUSE_SEMANT_INC_MSG, "Semantically Incorrect Message" },
 	{ GSM411_RP_CAUSE_INV_MAND_INF, "Invalid Mandatory Information" },
-	{ GSM411_RP_CAUSE_MSGTYPE_NOTEXIST, "Message Type non-existant" },
+	{ GSM411_RP_CAUSE_MSGTYPE_NOTEXIST, "Message Type non-existent" },
 	{ GSM411_RP_CAUSE_MSG_INCOMP_STATE, "Message incompatible with protocol state" },
 	{ GSM411_RP_CAUSE_IE_NOTEXIST, "Information Element not existing" },
 	{ GSM411_RP_CAUSE_PROTOCOL_ERR, "Protocol Error" },