crypto: fix pedantic compilation

crypto: fix pedantic compilation

/x86_64-native-linuxapp-gcc/include/rte_crypto.h:126:28:
error: ISO C forbids zero-size array ‘sym’ [-Werror=pedantic]
   struct rte_crypto_sym_op sym[0];
                            ^~~
Zero-size array is an extension to the language it cannot be replaced by a
empty size array i.e. [] because structure is inside a union.

Fixes: d2a4223c4c6d ("cryptodev: do not store pointer to op specific params")
Cc: stable@dpdk.org
Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>