Add missing include in s2n_certificate.c
Add missing include in s2n_certificate.c

string.h is needed for strnlen.

awslabs/s2n,gibson-compsci/s2n,alexeblee/s2n,colmmacc/s2n,raycoll/s2n,alexeblee/s2n,colmmacc/s2n,wcs1only/s2n,alexeblee/s2n,raycoll/s2n,raycoll/s2n,raycoll/s2n,PKRoma/s2n,gibson-compsci/s2n,awslabs/s2n,wcs1only/s2n,colmmacc/s2n,gibson-compsci/s2n,awslabs/s2n,awslabs/s2n,PKRoma/s2n,wcs1only/s2n,PKRoma/s2n,raycoll/s2n,gibson-compsci/s2n,wcs1only/s2n,awslabs/s2n,gibson-compsci/s2n,alexeblee/s2n,PKRoma/s2n,wcs1only/s2n,awslabs/s2n,PKRoma/s2n,raycoll/s2n,alexeblee/s2n,colmmacc/s2n,colmmacc/s2n,wcs1only/s2n,alexeblee/s2n,gibson-compsci/s2n,wcs1only/s2n,PKRoma/s2n,PKRoma/s2n,PKRoma/s2n,colmmacc/s2n,wcs1only/s2n