lpm: fix build of anonymous union initialization

lpm: fix build of anonymous union initialization

In SUSE11-SP3 i686 platform, with gcc 4.5.1, there is a
compile issue:
	rte_lpm.c: In function ‘add_depth_small_v20’:
	rte_lpm.c:778:7: error: unknown field ‘next_hop’
		specified in initializer

The root cause is gcc only allow anonymous union initialized
according to the field it is defined. But next_hop is defined
in different field when in different platform(Endian).

One solution is add if define in the code to avoid this issue,
but there is a simple way, initialize it separately later.

Fixes: afc5c914a083 ("lpm: fix big endian support")

Signed-off-by: Michael Qiu <michael.qiu@intel.com>