PCI: make isa_bridge Alpha-only

Since isa_bridge is neither assigned any value !NULL nor used on !Alpha, 
there's no reason for providing it.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/include/asm-alpha/pci.h b/include/asm-alpha/pci.h
index 4e115f3..85aa112 100644
--- a/include/asm-alpha/pci.h
+++ b/include/asm-alpha/pci.h
@@ -293,4 +293,6 @@
 #define IOBASE_ROOT_BUS		5
 #define IOBASE_FROM_HOSE	0x10000
 
+extern struct pci_dev *isa_bridge;
+
 #endif /* __ALPHA_PCI_H */