virpci: fix build on non-Linux

virPCIGetDeviceAddressFromSysfsLink() should return
virPCIDeviceAddressPtr, so return NULL in the stub instead of "-1".

src/util/virpci.c

@@ -2945,7 +2945,7 @@ virPCIDeviceAddressPtr
 virPCIGetDeviceAddressFromSysfsLink(const char *device_link ATTRIBUTE_UNUSED)
 {
     virReportError(VIR_ERR_INTERNAL_ERROR, "%s", _(unsupported));
-    return -1;
+    return NULL;
 }