usb: fix build warning in pci-quirks.h when CONFIG_PCI is not enabled

Fix warning when CONFIG_PCI is not enabled
(from commit 29636578).

drivers/usb/host/pci-quirks.h: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Moiz Sonasath <m-sonasath@ti.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>

drivers/usb/host/pci-quirks.h

@@ -13,6 +13,7 @@ void usb_enable_xhci_ports(struct pci_dev *xhci_pdev);
 void usb_disable_xhci_ports(struct pci_dev *xhci_pdev);
 void sb800_prefetch(struct device *dev, int on);
 #else
+struct pci_dev;
 static inline void usb_amd_quirk_pll_disable(void) {}
 static inline void usb_amd_quirk_pll_enable(void) {}
 static inline void usb_amd_dev_put(void) {}