KVM: allow host header to be included even for !CONFIG_KVM

The new context tracking subsystem unconditionally includes kvm_host.h
headers for the guest enter/exit macros.  This causes a compile
failure when KVM is not enabled.

Fix by adding an IS_ENABLED(CONFIG_KVM) check to kvm_host so it can
be included/compiled even when KVM is not enabled.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

include/linux/kvm_host.h

 #ifndef __KVM_HOST_H
 #define __KVM_HOST_H
 
+#if IS_ENABLED(CONFIG_KVM)
+
 /*
  * This work is licensed under the terms of the GNU GPL, version 2.  See
  * the COPYING file in the top-level directory.
@@ -1055,5 +1057,8 @@ static inline bool kvm_vcpu_eligible_for_directed_yield(struct kvm_vcpu *vcpu)
 }
 
 #endif /* CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT */
+#else
+static inline void __guest_enter(void) { return; }
+static inline void __guest_exit(void) { return; }
+#endif /* IS_ENABLED(CONFIG_KVM) */
 #endif
-