ctdb-common: Fix a warning in the pcap code

ctdb-common: Fix a warning in the pcap code

[173/416] Compiling ctdb/common/system_socket.c
../../common/system_socket.c: In function ‘ctdb_sys_read_tcp_packet’:
../../common/system_socket.c:1016:15: error: cast discards ‘const’ qualifier from pointer target type [-Werror=cast-qual]
 1016 |         eth = (struct ether_header *)buffer;
      |               ^

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
(cherry picked from commit 8b54587b1aed28aa2f3af7161a077aa9dd83894c)