Rename index to idx for main function in s2nc.c (#711)
Rename index to idx for main function in s2nc.c (#711)

Currently, some compilers would complain variable index shadows a global declaration.
Hence rename it.

Signed-off-by: Ke Liu <aa2dec28b167a66935faec9766cfae267cebad55@gmail.com>
awslabs/s2n,colmmacc/s2n,awslabs/s2n,gibson-compsci/s2n,colmmacc/s2n,raycoll/s2n,wcs1only/s2n,colmmacc/s2n,bpdavidson/s2n,PKRoma/s2n,raycoll/s2n,raycoll/s2n,awslabs/s2n,colmmacc/s2n,gibson-compsci/s2n,raycoll/s2n,wcs1only/s2n,alexeblee/s2n,bpdavidson/s2n,gibson-compsci/s2n,bpdavidson/s2n,raycoll/s2n,wcs1only/s2n,alexeblee/s2n,alexeblee/s2n,raycoll/s2n,wcs1only/s2n,PKRoma/s2n,PKRoma/s2n,PKRoma/s2n,wcs1only/s2n,PKRoma/s2n,alexeblee/s2n,bpdavidson/s2n,gibson-compsci/s2n,gibson-compsci/s2n,bpdavidson/s2n,bpdavidson/s2n,PKRoma/s2n,wcs1only/s2n,PKRoma/s2n,wcs1only/s2n,alexeblee/s2n,PKRoma/s2n,awslabs/s2n,gibson-compsci/s2n,awslabs/s2n,colmmacc/s2n,colmmacc/s2n,alexeblee/s2n,wcs1only/s2n,awslabs/s2n