build: Add internal copy of linux/gsmmux.h for older systems

Makefile.am

@@ -696,7 +696,7 @@ builtin_sources += plugins/allowed-apns.c
 sbin_PROGRAMS = src/ofonod
 
 src_ofonod_SOURCES = $(builtin_sources) $(gatchat_sources) \
-			linux/gpio.h src/ofono.ver \
+			linux/gsmmux.h linux/gpio.h src/ofono.ver \
 			src/main.c src/ofono.h src/log.c src/plugin.c \
 			src/modem.c src/common.h src/common.c \
 			src/manager.c src/dbus.c src/util.h src/util.c \

linux/gsmmux.h

@@ -0,0 +1,41 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _LINUX_GSMMUX_H
+#define _LINUX_GSMMUX_H
+
+#include <linux/if.h>
+#include <linux/ioctl.h>
+#include <linux/types.h>
+
+struct gsm_config
+{
+	unsigned int adaption;
+	unsigned int encapsulation;
+	unsigned int initiator;
+	unsigned int t1;
+	unsigned int t2;
+	unsigned int t3;
+	unsigned int n2;
+	unsigned int mru;
+	unsigned int mtu;
+	unsigned int k;
+	unsigned int i;
+	unsigned int unused[8];		/* Padding for expansion without
+					   breaking stuff */
+};
+
+#define GSMIOC_GETCONF		_IOR('G', 0, struct gsm_config)
+#define GSMIOC_SETCONF		_IOW('G', 1, struct gsm_config)
+
+struct gsm_netconfig {
+	unsigned int adaption;  /* Adaption to use in network mode */
+	unsigned short protocol;/* Protocol to use - only ETH_P_IP supported */
+	unsigned short unused2;
+	char if_name[IFNAMSIZ];	/* interface name format string */
+	__u8 unused[28];        /* For future use */
+};
+
+#define GSMIOC_ENABLE_NET      _IOW('G', 2, struct gsm_netconfig)
+#define GSMIOC_DISABLE_NET     _IO('G', 3)
+
+
+#endif