eal: clean up export of socket id variable

eal: clean up export of socket id variable

Theres no need to export this variable.  Its set and queried from an API call
that doesn't exist in the hot path.  Instead just export the rte_socket_id
symbol and make the variable private to protect it from type changes.  We should
do this with the other exported variables too, but I think its too late in the
release cycle to do that.

tested using distributor_autotest (which uses rte_socket_id), successfully.
Only tested on linux, as I don't currently have a bsd system spun up, but the
changes are symmetric, and should be fine

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Cunming Liang <cunming.liang@intel.com>
Acked-by: David Marchand <david.marchand@6wind.com>