New string in gvfs

[Bastien Nocera <hadess hadess net>]

Heya,

I added a new string to the ObexFTP gvfs backend:
USB support missing. Please contact your software vendor

Note that this string shouldn't be user visible, as we don't have any
HAL monitor changes made to detect those devices (so users would have
needed to generate the device address by hand).

Cheers