ctrl: use #define for TRAP id

We always use id = 0 when sending TRAP messages. Let's make this more
obvious by introducing appropriate define.

Change-Id: I33d7d4c6a1885a75a85d6f2f017430e0860b4126
diff --git a/include/osmocom/ctrl/control_cmd.h b/include/osmocom/ctrl/control_cmd.h
index 93055c7..276a7de 100644
--- a/include/osmocom/ctrl/control_cmd.h
+++ b/include/osmocom/ctrl/control_cmd.h
@@ -12,6 +12,7 @@
 #define CTRL_CMD_ERROR		-1
 #define CTRL_CMD_HANDLED	0
 #define CTRL_CMD_REPLY		1
+#define CTRL_CMD_TRAP_ID	"0"
 
 struct ctrl_handle;