Don't strdup the chld string for HFP

The chld string is of type const char * and is referring to a fixed
string in the telephony driver. I.e. there's no need to strdup it.
diff --git a/audio/headset.c b/audio/headset.c
index 97ae4fd..aaf7039 100644
--- a/audio/headset.c
+++ b/audio/headset.c
@@ -2793,7 +2793,7 @@
 	ag.features = features;
 	ag.indicators = indicators;
 	ag.rh = rh;
-	ag.chld = g_strdup(chld);
+	ag.chld = chld;
 
 	DBG("Telephony plugin initialized");