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: 5854a09 ("net/ipv4: Use __DECLARE_FLEX_ARRAY() helper")
Signed-off-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Cc: Gustavo A. R. Silva <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: anakryiko

include/uapi/linux/in.h

@@ -20,6 +20,7 @@
 #define _UAPI_LINUX_IN_H
 
 #include <linux/types.h>
+#include <linux/stddef.h>
 #include <linux/libc-compat.h>
 #include <linux/socket.h>