uapi: add missing ip/ipv6 header dependencies for linux/stddef.h

Since commit 58e0be1ef6 ("net: use struct_group to copy ip/ipv6
header addresses"), ip and ipv6 headers started to use the __struct_group
definition, which is defined at include/uapi/linux/stddef.h. However,
linux/stddef.h isn't explicitly included in include/uapi/linux/{ip,ipv6}.h,
which breaks build of xskxceiver bpf selftest if you install the uapi
headers in the system:

$ make V=1 xskxceiver -C tools/testing/selftests/bpf
...
make: Entering directory '(...)/tools/testing/selftests/bpf'
gcc -g -O0 -rdynamic -Wall -Werror (...)
In file included from xskxceiver.c:79:
/usr/include/linux/ip.h:103:9: error: expected specifier-qualifier-list before ‘__struct_group’
  103 |         __struct_group(/* no tag */, addrs, /* no attrs */,
      |         ^~~~~~~~~~~~~~
...

Include the missing <linux/stddef.h> dependency in ip.h and do the
same for the ipv6.h header.

Fixes: 58e0be1ef6 ("net: use struct_group to copy ip/ipv6 header addresses")
Signed-off-by: Herton R. Krzesinski <herton@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/ip.h

@@ -18,6 +18,7 @@
 #ifndef _UAPI_LINUX_IP_H
 #define _UAPI_LINUX_IP_H
 #include <linux/types.h>
+#include <linux/stddef.h>
 #include <asm/byteorder.h>
 
 #define IPTOS_TOS_MASK		0x1E

include/uapi/linux/ipv6.h

@@ -4,6 +4,7 @@
 
 #include <linux/libc-compat.h>
 #include <linux/types.h>
+#include <linux/stddef.h>
 #include <linux/in6.h>
 #include <asm/byteorder.h>