examples/performance-thread: restrict to x86_64
examples/performance-thread: restrict to x86_64

Performance-thread sample app is only supported for x86_64 targets,
so this commit adds a check to avoid compilation on other targets.

Signed-off-by: Pablo de Lara <28355d9cfe41bb8148aea0fb24f47aae8f4217a1@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk