x86/vsdo: Fix build on PARAVIRT_CLOCK=y, KVM_GUEST=n

arch/x86/built-in.o: In function `arch_setup_additional_pages':
 (.text+0x587): undefined reference to `pvclock_pvti_cpu0_va'

KVM_GUEST selects PARAVIRT_CLOCK, so we can make pvclock_pvti_cpu0_va depend
on KVM_GUEST.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Tested-by: Borislav Petkov <bp@alien8.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Link: http://lkml.kernel.org/r/444d38a9bcba832685740ea1401b569861d09a72.1451446564.git.luto@kernel.orgSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/include/asm/pvclock.h

@@ -4,7 +4,7 @@
 #include <linux/clocksource.h>
 #include <asm/pvclock-abi.h>
 
-#ifdef CONFIG_PARAVIRT_CLOCK
+#ifdef CONFIG_KVM_GUEST
 extern struct pvclock_vsyscall_time_info *pvclock_pvti_cpu0_va(void);
 #else
 static inline struct pvclock_vsyscall_time_info *pvclock_pvti_cpu0_va(void)