xen: Move xen_have_vector_callback definition to enlighten.c

Commit 84d582d236dc ("xen: Revert commits da72ff5bfcb0 and
72a9b186292d") defined xen_have_vector_callback in enlighten_hvm.c.
Since guest-type-neutral code refers to this variable this causes
build failures when CONFIG_XEN_PVHVM is not defined.

Moving xen_have_vector_callback definition to enlighten.c resolves
this issue.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

arch/x86/xen/enlighten.c

@@ -57,6 +57,9 @@ EXPORT_SYMBOL_GPL(xen_start_info);
 
 struct shared_info xen_dummy_shared_info;
 
+__read_mostly int xen_have_vector_callback;
+EXPORT_SYMBOL_GPL(xen_have_vector_callback);
+
 /*
  * Point at some empty memory to start with. We map the real shared_info
  * page as soon as fixmap is up and running.

arch/x86/xen/enlighten_hvm.c

@@ -18,9 +18,6 @@
 #include "mmu.h"
 #include "smp.h"
 
-__read_mostly int xen_have_vector_callback;
-EXPORT_SYMBOL_GPL(xen_have_vector_callback);
-
 void __ref xen_hvm_init_shared_info(void)
 {
 	int cpu;