usb1._libusb1: Mark libusb_control_setup as packed

Following a libusb commit: which states that libusb relies on this struct
not having any padding but until now it was left to the compiler to decide.
Apparently no compiler (nor ctypes) insert padding as otherwise it would
have broken all asynchronous control transfers, but better be explicit.

usb1/_libusb1.py

@@ -942,6 +942,7 @@ class libusb_config_descriptor(Structure):
 libusb_config_descriptor_p_p = POINTER(libusb_config_descriptor_p)
 
 class libusb_control_setup(Structure):
+    _pack_ = 1
     _fields_ = [
         ('bmRequestType', c_uint8),
         ('bRequest', c_uint8),