missing include asm/paravirt.h in cputime.c

Add include asm/paravirt.h to cputime.c, as steal_account_process_tick
calls paravirt_steal_clock, which is defined in asm/paravirt.h.

The ifdef CONFIG_PARAVIRT is necessary because not all archs have an
asm/paravirt.h to include.

The reason why currently cputime.c compiles, even though include
<asm/paravirt.h> is missing, is that on x86 asm/paravirt.h is included
by one of the other headers included in kernel/sched/cputime.c:

On arm and arm64, where I am about to introduce asm/paravirt.h and
stolen time support, without #include <asm/paravirt.h> in cputime.c, I
would get an error.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: NPeter Zijlstra (Intel) <peterz@infradead.org>

kernel/sched/cputime.c

@@ -5,6 +5,9 @@
 #include <linux/static_key.h>
 #include <linux/context_tracking.h>
 #include "sched.h"
+#ifdef CONFIG_PARAVIRT
+#include <asm/paravirt.h>
+#endif
 
 
 #ifdef CONFIG_IRQ_TIME_ACCOUNTING