fix various compiler warnings (on FreeBSD-11.0)

FreeBSD 11.0 uses clang version 3.8.0 which spits various warnings
during libosmocore compilation.  Let's clean this up a bit.

Change-Id: Ic14572e6970bd0b8916604fabf807f1608fa07e5
diff --git a/src/ctrl/control_if.c b/src/ctrl/control_if.c
index df39486..b4afd55 100644
--- a/src/ctrl/control_if.c
+++ b/src/ctrl/control_if.c
@@ -131,7 +131,7 @@
 		return -ENOMEM;
 
 	cmd->id = "0"; /* It's a TRAP! */
-	cmd->variable = name;
+	cmd->variable = (char *) name;
 	cmd->reply = value;
 	r = ctrl_cmd_send_to_all(ctrl, cmd);
 	talloc_free(cmd);