arpd: use designated initializers for msghdr structure

arpd: use designated initializers for msghdr structure

This patch fixes the following error:

arpd.c:442:17: error: initialization of 'int' from 'void *' makes integer from pointer without a cast [-Wint-conversion]
  442 |                 NULL,   0,

raised by Buildroot autobuilder [1].

In the case in question, the analysis of socket.h [2] containing the
msghdr structure shows that it has been modified with the addition of
padding fields, which cause the compilation error. The use of designated
initializers allows the issue to be fixed.

struct msghdr {
	void *msg_name;
	socklen_t msg_namelen;
	struct iovec *msg_iov;
	int __pad1;
	int msg_iovlen;
	int __pad1;
	void *msg_control;
	int __pad2;
	socklen_t msg_controllen;
	int __pad2;
	int msg_flags;
};

[1] http://autobuild.buildroot.org/results/e4cdfa38ae9578992f1c0ff5c4edae3cc0836e3c/
[2] iproute2/host/mips64-buildroot-linux-musl/sysroot/usr/include/sys/socket.h

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>