libndp: fix incompatible pointer types with gcc14 and musl

[oreo639]

When compiling with gcc14 and musl, the following error is produced:

libndp.c: In function 'mysendto6':
libndp.c:212:50: error: passing argument 5 of 'sendto' from incompatible pointer type [-Wincompatible-pointer-types]
  212 |         ret = sendto(sockfd, buf, buflen, flags, &sin6, sizeof(sin6));
      |                                                  ^~~~~
      |                                                  |
      |                                                  struct sockaddr_in6 *
In file included from libndp.c:27:
/usr/include/sys/socket.h:343:49: note: expected 'const struct sockaddr *' but argument is of type 'struct sockaddr_in6 *'
  343 | ssize_t sendto (int, const void *, size_t, int, const struct sockaddr *, socklen_t);
      |                                                 ^~~~~~~~~~~~~~~~~~~~~~~

In POSIX, sendto() takes a sockaddr pointer:
https://pubs.opengroup.org/onlinepubs/009604499/functions/sendto.html

While glibc uses the gcc __transparent_union__ extension to mark them as compatible types, musl does not, as such we need to explicitly cast the pointer to tell the compiler that it is fine.

#25

[oreo639]

Submitted to the mailing list (waiting moderator approval): https://lists.fedorahosted.org/archives/list/libndp@lists.fedorahosted.org/latest