package/rt-tests: add syscall-number for sched_(gs)etattr() for SH4

package/rt-tests: add syscall-number for sched_(gs)etattr() for SH4

The dependency on kernel headers >= 3.14 was added by
b9fa6c00b1213fd038f2aed52057f22c7e18fd4b since
librttest requires __NR_sched_setattr and
__NR_sched_getattr syscalls (rt-sched).

But these syscalls have just been added to kernel 4.8 for SH4.

Fixes:
http://autobuild.buildroot.net/results/67fc16366717f34df492683a14ecd23376e23110

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>