Fix -Wcast-qual warnings

Fix -Wcast-qual warnings

Without this change I get the following -Werror build failure when building
samba on macOS:
```
../../third_party/socket_wrapper/socket_wrapper.c:5420:15: error: cast from 'const struct cmsghdr *' to 'unsigned char *' drops const qualifier [-Werror,-Wcast-qual]
        __fds_in.p = CMSG_DATA(cmsg);
                     ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/socket.h:631:51: note: expanded from macro 'CMSG_DATA'
#define CMSG_DATA(cmsg)         ((unsigned char *)(cmsg) + \
                                                  ^
```

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>