bug#79299: nproc-quota test failure

Collin Funk wrote:
> The CentOS 7 one fails because SCHED_DEADLINE is not defined by
> sched.h. But it is defined in linux/sched.h. The second patch, which is
> for Gnulib, fixed it in my CentOS 7 container.

This patch would be fine to apply in Gnulib, except that the rationale
(why do we include <linux/sched.h>?) belongs in the code, not in the commit
message. In the commit message, on the other hand, I like to see an indication
of the symptom and severity (compilation error? link error? warning?
missed optimization?), e.g. "compilation error with older glibc versions".

Bruno