sched/core: Fix build paravirt build on arm and arm64

Commit 004172bd ("sched/core: Remove unnecessary #include headers")
removed the inclusion of asm/paravirt.h which is used to get
declarations of paravirt_steal_rq_enabled and paravirt_steal_clock.

It is implicitly included on x86 but not on arm and arm64 breaking the
build if paravirtualization is used.  Since things from that header are
used directly fix the build by putting the direct inclusion back.
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sched/core.c

@@ -23,6 +23,9 @@
 
 #include <asm/switch_to.h>
 #include <asm/tlb.h>
+#ifdef CONFIG_PARAVIRT
+#include <asm/paravirt.h>
+#endif
 
 #include "sched.h"
 #include "../workqueue_internal.h"