net: sockets: Prevent compiler error if warnings being treated as errors

If gcc compiler option -Werror is used the warning,

declared inside parameter list will not be visible outside of this
definition or declaration [-Werror]

is treated as error, for

sockets_internal.h:18:28: ‘struct net_context’
sockets_internal.h:19:32: ‘struct zsock_pollfd’
fdtable.h:108:17: ‘struct k_mutex’

Signed-off-by: Christoph Schnetzler <christoph.schnetzler@husqvarnagroup.com>