Fix build with older gcc

Fix build with older gcc

My commit af1c98e4 broke the build on RHEL-6:
vircgrouptest.c: In function 'testCgroupGetPercpuStats':
vircgrouptest.c:566: error: nested extern declaration of
'_gl_verify_function2' [-Wnested-externs]

The only thing that needs checking is that the array size
is at least EXPECTED_NCPUS, to prevent access beyond the array.

We can ensure the minimum size also by specifying the array
size upfront.