src/libcephfs_proxy/CMakeLists.txt: add -Wno-gnu-pointer-arith

src/libcephfs_proxy/CMakeLists.txt: add -Wno-gnu-pointer-arith

```
Building C object src/libcephfs_proxy/CMakeFiles/cephfs_proxy.dir/proxy_link.c.o
/home/./ceph/src/libcephfs_proxy/proxy_link.c:198:10: warning: arithmetic on a pointer to void is a GNU extension [-Wgnu-pointer-arith]
  198 |                 buffer += len;
      |                 ~~~~~~ ^
/home/./ceph/src/libcephfs_proxy/proxy_link.c:232:18: warning: arithmetic on a pointer to void is a GNU extension [-Wgnu-pointer-arith]
  232 |                         iov->iov_base += len;
      |                         ~~~~~~~~~~~~~ ^
/home/./ceph/src/libcephfs_proxy/proxy_link.c:268:18: warning: arithmetic on a pointer to void is a GNU extension [-Wgnu-pointer-arith]
  268 |                         iov->iov_base += len;
      |                         ~~~~~~~~~~~~~ ^
/home/./ceph/src/libcephfs_proxy/proxy_link.c:328:17: warning: arithmetic on a pointer to void is a GNU extension [-Wgnu-pointer-arith]
  328 |                 iov->iov_base += sizeof(proxy_link_req_t);
      |                 ~~~~~~~~~~~~~ ^
/home/./ceph/src/libcephfs_proxy/proxy_link.c:391:17: warning: arithmetic on a pointer to void is a GNU extension [-Wgnu-pointer-arith]
  391 |                 iov->iov_base += sizeof(proxy_link_ans_t);
      |                 ~~~~~~~~~~~~~ ^
5 warnings generated.
```

Signed-off-by: Matan Breizman <mbreizma@redhat.com>