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/gsm_04_08_gprs.c b/src/gsm/gsm_04_08_gprs.c
index 608fa8c..8032593 100644
--- a/src/gsm/gsm_04_08_gprs.c
+++ b/src/gsm/gsm_04_08_gprs.c
@@ -65,7 +65,7 @@
 	{ GMM_CAUSE_SEM_INCORR_MSG,	"Semantically incorrect message" },
 	{ GMM_CAUSE_INV_MAND_INFO, "Invalid mandatory information" },
 	{ GMM_CAUSE_MSGT_NOTEXIST_NOTIMPL,
-			"Message type non-existant or not implemented" },
+			"Message type non-existent or not implemented" },
 	{ GMM_CAUSE_MSGT_INCOMP_P_STATE,
 			"Message type not compatible with protocol state" },
 	{ GMM_CAUSE_IE_NOTEXIST_NOTIMPL,
@@ -105,7 +105,7 @@
 	{ GSM_CAUSE_SEM_INCORR_MSG, "Semantically incorrect message" },
 	{ GSM_CAUSE_INV_MAND_INFO, "Invalid mandatory information" },
 	{ GSM_CAUSE_MSGT_NOTEXIST_NOTIMPL,
-			"Message type non-existant or not implemented" },
+			"Message type non-existent or not implemented" },
 	{ GSM_CAUSE_MSGT_INCOMP_P_STATE,
 			"Message type not compatible with protocol state" },
 	{ GSM_CAUSE_IE_NOTEXIST_NOTIMPL,