lecroy-logicstudio: Fix some compiler warnings on MinGW.

lecroy-logicstudio: Fix some compiler warnings on MinGW.

src/hardware/lecroy-logicstudio/protocol.c: In function 'handle_fetch_samples_done':
src/hardware/lecroy-logicstudio/protocol.c:261:3: warning: passing argument 6 of 'libusb_fill_bulk_transfer' from incompatible pointer type
   recv_bulk_transfer, (void *)sdi, USB_TIMEOUT_MS);
   ^
In file included from ./src/libsigrok-internal.h:31:0,
                 from src/hardware/lecroy-logicstudio/protocol.h:26,
                 from src/hardware/lecroy-logicstudio/protocol.c:23:
/home/uwe/sr_mingw/include/libusb-1.0/libusb.h:1546:20: 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,
                    ^
src/hardware/lecroy-logicstudio/protocol.c: In function 'fetch_samples_async':
src/hardware/lecroy-logicstudio/protocol.c:314:4: warning: passing argument 6 of 'write_registers_async' from incompatible pointer type
    handle_fetch_samples_done);
    ^
src/hardware/lecroy-logicstudio/protocol.c:200:12: note: expected 'libusb_transfer_cb_fn' but argument is of type 'void (*)(struct libusb_transfer *)'
 static int write_registers_async(const struct sr_dev_inst *sdi,
            ^
src/hardware/lecroy-logicstudio/protocol.c: In function 'lls_start_acquisition':
src/hardware/lecroy-logicstudio/protocol.c:1122:3: warning: passing argument 6 of 'libusb_fill_interrupt_transfer' from incompatible pointer type
   recv_intr_transfer, (void *) sdi, USB_TIMEOUT_MS);
   ^
In file included from ./src/libsigrok-internal.h:31:0,
                 from src/hardware/lecroy-logicstudio/protocol.h:26,
                 from src/hardware/lecroy-logicstudio/protocol.c:23:
/home/uwe/sr_mingw/include/libusb-1.0/libusb.h:1602:20: note: expected 'libusb_transfer_cb_fn' but argument is of type 'void (*)(struct libusb_transfer *)'
 static inline void libusb_fill_interrupt_transfer(
                    ^