x86, k8: Fix build error when K8_NB is disabled

K8_NB depends on PCI and when the last is disabled (allnoconfig) we fail
at the final linking stage due to missing exported num_k8_northbridges.
Add a header stub for that.
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
LKML-Reference: <20100503183036.GJ26107@aftab>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: <stable@kernel.org>

arch/x86/include/asm/k8.h

@@ -16,11 +16,16 @@ extern int k8_numa_init(unsigned long start_pfn, unsigned long end_pfn);
 extern int k8_scan_nodes(void);
 
 #ifdef CONFIG_K8_NB
+extern int num_k8_northbridges;
+
 static inline struct pci_dev *node_to_k8_nb_misc(int node)
 {
 	return (node < num_k8_northbridges) ? k8_northbridges[node] : NULL;
 }
+
 #else
+#define num_k8_northbridges 0
+
 static inline struct pci_dev *node_to_k8_nb_misc(int node)
 {
 	return NULL;