cmake: disable pedantic

cmake: disable pedantic

This flag breaks compilation with ubus on aarch64/arm/x86.
The only platform which compiles successfully is mips.

/home/runner/work/odhcp6c/odhcp6c/build/include/libubox/utils.h:126:31: error: invalid application of ‘sizeof’ to a void type [-Werror=pointer-arith]
  126 |         (sizeof(int) == sizeof(*(1 ? ((void*)((long)(x) * 0l)) : (int*)1)))
      |                               ^
/home/runner/work/odhcp6c/odhcp6c/build/include/libubox/utils.h:140:31: note: in expansion of macro ‘__is_constant’
  140 |         __builtin_choose_expr(__is_constant(x),                         \
      |                               ^~~~~~~~~~~~~
/home/runner/work/odhcp6c/odhcp6c/build/include/libubox/utils.h:167:24: note: in expansion of macro ‘__eval_safe’
  167 | #define be32_to_cpu(x) __eval_safe(__constant_swap32, x)
      |                        ^~~~~~~~~~~

Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/odhcp6c/pull/109
Signed-off-by: Álvaro Fernández Rojas <[email protected]>