siit: fix compilation warning

siit: fix compilation warning

Fix compilation warning for stack limit and variable length array.
Fix compilation warning:
  CC [M]  /home/ansuel/openwrt-ansuel/openwrt/build_dir/target-aarch64_cortex-a53_musl/linux-ipq807x_generic/siit-1.2/siit.o
../siit-1.2/siit.c: In function 'ip4_fragment':
../siit-1.2/siit.c:988:9: error: ISO C90 forbids variable length array 'buff' [-Werror=vla]
  988 |         char buff[FRAG_BUFF_SIZE+hdr_len]; /* buffer to form new fragment packet */
      |         ^~~~
../siit-1.2/siit.c: In function 'siit_xmit':
../siit-1.2/siit.c:1359:1: error: the frame size of 2144 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]
 1359 | }
      | ^
cc1: all warnings being treated as errors

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>