hlr_ussd.h: use proper libc headers

Change-Id: I2b9485be08c6cbf188ed1f4059ff28ab65c61dbf
diff --git a/src/hlr_ussd.h b/src/hlr_ussd.h
index 67e5d0d..a190dc6 100644
--- a/src/hlr_ussd.h
+++ b/src/hlr_ussd.h
@@ -1,6 +1,7 @@
 #pragma once
 
-#include <stdint.h>
+#include <stdbool.h>
+
 #include <osmocom/core/linuxlist.h>
 #include <osmocom/gsm/gsup.h>
 #include "gsup_server.h"