add missing endian.h in gsm_23_041.h

Change-Id: I593cc5e8272469b570559206bb02b6e79797340b
diff --git a/include/osmocom/gsm/protocol/gsm_23_041.h b/include/osmocom/gsm/protocol/gsm_23_041.h
index c75c088..e60c324 100644
--- a/include/osmocom/gsm/protocol/gsm_23_041.h
+++ b/include/osmocom/gsm/protocol/gsm_23_041.h
@@ -1,5 +1,7 @@
 #pragma once
 
+#include <osmocom/core/endian.h>
+
 /* Section 9.4.1.2: GSM Message Format */
 struct gsm23041_msg_param_gsm {
 	uint16_t serial_nr;