net/ipv4: Fix linux/in.h header dependencies

__DECLARE_FLEX_ARRAY is defined in include/uapi/linux/stddef.h but
doesn't seem to be explicitly included from include/uapi/linux/in.h,
which breaks BPF selftests builds (once we sync linux/stddef.h into
tools/include directory in the next patch). Fix this by explicitly
including linux/stddef.h.

Given this affects BPF CI and bpf tree, targeting this for bpf tree.

Fixes: 5854a09b ("net/ipv4: Use __DECLARE_FLEX_ARRAY() helper")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/bpf/20221102182517.2675301-1-andrii@kernel.org