eal: fix keep alive header for C++

eal: fix keep alive header for C++

When built in a C++ application, the keepalive include fails:

rte_keepalive.h:142:41: error: ‘ALIVE’ was not declared in this scope
  keepcfg->state_flags[rte_lcore_id()] = ALIVE;
                                         ^
C++ requires to use a scope operator to access an enum inside a struct.
There was also a namespace issue for the values (no RTE prefix).
The solution is to move the struct and related code out of the header file.

Fixes: 75583b0d1efd ("eal: add keep alive monitoring")
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Remy Horton <remy.horton@intel.com>