Add human-readable name of SGSN_AUTH_AUTHENTICATE

In commit 4adb136da6 we introduced
a new authentication state SGSN_AUTH_AUTHENTICATE, but we didn't
add that to auth_state_names[] resulting in log messages printing
it abut 'unknown 0x1' rather than something more useful.

openbsc/src/gprs/sgsn_auth.c

@@ -30,6 +30,7 @@ const struct value_string auth_state_names[] = {
 	{ SGSN_AUTH_ACCEPTED,	"accepted"},
 	{ SGSN_AUTH_REJECTED,	"rejected"},
 	{ SGSN_AUTH_UNKNOWN,	"unknown"},
+	{ SGSN_AUTH_AUTHENTICATE, "authenticate" },
 	{ 0, NULL }
 };