am7xxx: specify LIBUSB_CALL for send_data_async_complete_cb()

send_data_async_complete_cb() is used as a callback for
libusb_fill_bulk_transfer() and it is expected to be of type
'libusb_transfer_cb_fn' which is defined using LIBUSB_CALL.

LIBUSB_CALL is only used on Windows and not having it results in the
following warning when compiling for Windows:

  .../libam7xxx/src/am7xxx.c: In function ‘send_data_async’:
  .../libam7xxx/src/am7xxx.c:508:7: warning: passing argument 6 of ‘libusb_fill_bulk_transfer’ from incompatible pointer type [-Wincompatible-pointer-types]
         send_data_async_complete_cb, dev, 0);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
  In file included from /home/ao2/Proj/picoProjector/libam7xxx/src/am7xxx.c:24:0:
  .../libam7xxx/build/libusb-1.0.21/include/libusb-1.0/libusb.h:1553:20: note: expected ‘libusb_transfer_cb_fn’ but argument is of type ‘void (*)(struct libusb_transfer *)’
   static inline void libusb_fill_bulk_transfer(struct libusb_transfer *transfer,
                      ^~~~~~~~~~~~~~~~~~~~~~~~~