server.c: fix clang warning

[ilg-ul]

/Users/ilg/Work/openocd/openocd.git/src/server/server.c:305:22: error: incompatible pointer types passing 'struct sockaddr_in *' to
parameter of type 'struct sockaddr *' [-Werror,-Wincompatible-pointer-types]
getsockname(c->fd, &addr_in, &addr_in_size);
^~~~~~~~
/usr/include/sys/socket.h:687:50: note: passing argument to parameter here
int     getsockname(int, struct sockaddr * __restrict, socklen_t * __restrict)