test/pmd_perf: declare variables as static
test/pmd_perf: declare variables as static

Some variables in the PMD perf test were declared as global,
but they are only used in this test, so they should actually
be declared as static.

Fixes: 002ade70e933 ("app/test: measure cycles per packet in Rx/Tx")

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