Add _GNU_SOURCE to s2n_certificate.c
Add _GNU_SOURCE to s2n_certificate.c

Need this for platforms with glibc < 2.10

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