build: Allow ipsec-mb plugin to build with libipsec_mb 0.55

build: Allow ipsec-mb plugin to build with libipsec_mb 0.55

The 0.55 version of libipsec_mb does not support the chacha functions
used in the plugin.

The missing symobls are:
ipsecmb_ops_chacha_poly
ipsecmb_ops_chacha_poly_chained
IMB_CIPHER_DIRECTION

Check for ipsecmb_ops_chacha_poly() and conditionalise the chacha code
in the plugin on this.

ipsec_mb 0.55 is the version currently found in Debian Stable (bullseye)

Type: make
Signed-off-by: Nick Brown <nickbroon@gmail.com>
Change-Id: I88c962ac4f99a58b5cd61fb9b75f692e27d4ec30