Fix compile-time issues for a few _struct_compat_*_u() functions.

Fix compile-time issues for a few _struct_compat_*_u() functions.

* tapset/linux/aux_syscalls.stp: Fix _struct_compat_itimerspec_u(),
  and _struct_itimerval_u() so that they compile on non CONFIG_COMPAT
  systems. Fix _struct_sched_attr_u() so that it compiles on kernels
  not having struct sched_attr, i.e. when kernel_v < "3.14".
* testsuite/buildok/aux_syscalls-embedded.stp: Remove unneeded
  constraint for _struct_sched_attr_u() testing.