ltq-ptm: adsl: fix 6.12 kernel build errors on lantiq/xway

Fix incomplete type error by including the correct header.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/18744
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit 2e00140e63c97f469d472212189cec1c0c28ebc4)
Signed-off-by: Nora Matthias Schiffer <neocturne@universe-factory.net>
Link: https://github.com/openwrt/openwrt/pull/20440
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 0d02cf9e3464c9f415877bf0a7ce6416a498cd18)

package/kernel/lantiq/ltq-ptm/patches/104-add-missing-header-mod_devicetable.h.patch

@@ -0,0 +1,25 @@
+From: Shiji Yang <yangshiji66@outlook.com>
+Date: Fri, 9 May 2025 21:16:08 +0800
+Subject: [PATCH] add missing header "mod_devicetable.h"
+
+Fix compilation error:
+
+/home/db/owrt/build_dir/target-mips_24kc_musl/linux-lantiq_xway/ltq-ptm-danube/ltq-ptm/ifxmips_ptm_adsl.c:1433:34: error: array type has incomplete element type 'struct of_device_id'
+ 1433 | static const struct of_device_id ltq_ptm_match[] = {
+      |                                  ^~~~~~~~~~~~~
+
+Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
+---
+ ifxmips_ptm_adsl.c | 1 +
+ 1 file changed, 1 insertion(+)
+
+--- a/ifxmips_ptm_adsl.c
++++ b/ifxmips_ptm_adsl.c
+@@ -43,6 +43,7 @@
+ #include <linux/ioctl.h>
+ #include <linux/etherdevice.h>
+ #include <linux/interrupt.h>
++#include <linux/mod_devicetable.h>
+ #include <linux/netdevice.h>
+ #include <linux/platform_device.h>
+ #include <linux/of_device.h>