samples/bpf: Add missing option to xdpsock usage

Commit 743e568c (samples/bpf: Add a "force" flag to XDP samples)
introduced the '-F' option but missed adding it to the usage() and the
'long_option' array.

Fixes: 743e568c

 (samples/bpf: Add a "force" flag to XDP samples)
Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191114162847.221770-2-andre.guedes@intel.com