eal: fix C++ app build

eal: fix C++ app build

 * Forward declaration of enum in C++ requires explicit underlying
   type definitions.

 * This fixes the issue at:
   http://dpdk.org/ml/archives/dev/2015-April/017065.html

include/generic/rte_cpuflags.h:50:6:
error: use of enum ‘rte_cpu_flag_t’ without previous declaration
 enum rte_cpu_flag_t;

include/generic/rte_cpuflags.h:55:6:
error: use of enum ‘cpu_register_t’ without previous declaration
 enum cpu_register_t;

Signed-off-by: Joongi Kim <joongi@an.kaist.ac.kr>
[Thomas: fix extended to ppc and tile]