service: fix build with gcc 4.9

service: fix build with gcc 4.9

Some compilers detect this error:

    error: ‘ids[0]’ may be used uninitialized in this function
    ret = rte_service_map_lcore_set(i, ids[lcore_iter], 1);

It can be reproduced very easily on Fedora 21 with
gcc-4.9.2-6.fc21.x86_64.

Fixes: 21698354c832 ("service: introduce service cores concept")
Cc: stable@dpdk.org
Signed-off-by: Yi Yang <yi.y.yang@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>