libusb.h: Ensure that struct libusb_control_transfer is not padded

libusb.h: Ensure that struct libusb_control_transfer is not padded

There is nothing explicitly preventing the compiler from adding any sort
of padding to the libusb_control_transfer structure. It does not seem
that any sane compiler would do so, but there is library functionality
that depends on this not happening. Address this by explicitly
instructing the compiler to pack the structure.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>