xdp-tools: add patch to fix stddef.h build issue

xdp-tools: add patch to fix stddef.h build issue

Add a patch that avoids including <stddef.h> in BPF headers, fixing
build failures on OpenWrt toolchains where the header is unavailable:

  In file included from xdpfilt_dny_udp.c:10:
  In file included from ./xdpfilt_prog.h:24:
  ../lib/../headers/xdp/parsing_helpers.h:18:10: fatal error: 'stddef.h' file not found
    18 | #include <stddef.h>
        |          ^~~~~~~~~~
  1 error generated.
  make[5]: *** [../lib/common.mk:111: xdpfilt_dny_udp.o] Error 1
  make[4]: *** [Makefile:40: xdp-filter] Error 2

Link: https://github.com/openwrt/openwrt/pull/21972
Signed-off-by: Nick Hainke <vincent@systemli.org>