KVM: Export include/linux/kvm.h only if $ARCH actually supports KVM

Currently, make headers_check barfs due to <asm/kvm.h>, which <linux/kvm.h>
includes, not existing.  Rather than add a zillion <asm/kvm.h>s, export kvm.h
only if the arch actually supports it.

Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index fb3eea3..d289cfc 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -107,6 +107,7 @@
 	bool
 	default y
 
+select HAVE_KVM
 
 config ZONE_DMA32
 	bool