Orion5x: Fix warning: struct pci_dev declared inside paramter list

Orion5x board files which don't have PCI give warnings:

arch/arm/mach-orion5x/common.h:54:38: warning: 'struct pci_dev' declared
inside parameter list.

Add a forward declaration in the header file, which is the pattern
used for other PCI structures.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
diff --git a/arch/arm/mach-orion5x/common.h b/arch/arm/mach-orion5x/common.h
index 2e6454c..31bab92 100644
--- a/arch/arm/mach-orion5x/common.h
+++ b/arch/arm/mach-orion5x/common.h
@@ -45,6 +45,7 @@
  */
 struct pci_bus;
 struct pci_sys_data;
+struct pci_dev;
 
 void orion5x_pcie_id(u32 *dev, u32 *rev);
 void orion5x_pci_disable(void);