added eap=gtc option to ipsec.conf man page

src/starter/ipsec.conf.5

@@ -356,7 +356,9 @@ selected. Acceptable values are
 .B aka
 for EAP-AKA,
 .B sim
-for EAP-SIM and
+for EAP-SIM,
+.B gtc
+for EAP-GTC, and
 .B md5
 for EAP-MD5.
 Additionally, IANA assigned EAP method numbers are accepted, or a definition