pci: fix missing extern for pci_testdev()

Suggested-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

lib/pci.h

@@ -43,7 +43,7 @@ extern bool pci_bar_is_valid(pcidevaddr_t dev, int bar_num);
 extern void pci_bar_print(pcidevaddr_t dev, int bar_num);
 extern void pci_dev_print_id(pcidevaddr_t dev);
 
-int pci_testdev(void);
+extern int pci_testdev(void);
 
 /*
  * pci-testdev is a driver for the pci-testdev qemu pci device. The