lpm: fix ARM big endian build

lpm: fix ARM big endian build

Compiling on ARM BE using Linaro toolchain caused following
error/warnings.

rte_lpm.c: In function ‘add_depth_big_v20’:
rte_lpm.c:911:4: error: braces around scalar initializer [-Werror]
    { .group_idx = (uint8_t)tbl8_group_index, },
    ^
rte_lpm.c:911:4: note: (near initialization for
	‘new_tbl24_entry.depth’)
rte_lpm.c:911:6:error: field name not in record or union initializer
    { .group_idx = (uint8_t)tbl8_group_index, },
      ^
rte_lpm.c:911:6: note: (near initialization for
	‘new_tbl24_entry.depth’)
rte_lpm.c:914:13: error: initialized field overwritten
	[-Werror=override-init]
    .depth = 0,

Fixes: dc81ebbacaeb ("lpm: extend IPv4 next hop field")
Cc: stable@dpdk.org
Signed-off-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>