KVM: Fix compilation on non-x86

Commit 84b058d broke compilation for KVM on non-x86 targets, which
don't have KVM_CAP_IRQ_ROUTING defined.

Fix by not using the unavailable constant when it's not around.

Signed-off-by: Alexander Graf <agraf@suse.de>
diff --git a/kvm-all.c b/kvm-all.c
index 7d4e544..0b87658 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -1306,7 +1306,11 @@
 
 int kvm_has_gsi_routing(void)
 {
+#ifdef KVM_CAP_IRQ_ROUTING
     return kvm_check_extension(kvm_state, KVM_CAP_IRQ_ROUTING);
+#else
+    return false;
+#endif
 }
 
 int kvm_allows_irq0_override(void)