Windows: fix compiler warnings

libusb/os/poll_windows.c:176:5: warning: no previous prototype for '_fd_to_index_and_lock' [-Wmissing-prototypes]
libusb/os/poll_windows.c:197:13: warning: no previous prototype for 'create_overlapped' [-Wmissing-prototypes]
libusb/os/poll_windows.c:211:6: warning: no previous prototype for 'free_overlapped' [-Wmissing-prototypes]
libusb/os/poll_windows.c:223:6: warning: no previous prototype for 'reset_overlapped' [-Wmissing-prototypes]
libusb/os/poll_windows.c:401:6: warning: no previous prototype for '_free_index' [-Wmissing-prototypes]
Windows: remove unused function

libusb/os/poll_windows.c:223:13: warning: 'reset_overlapped' defined but not used [-Wunused-function]
Windows: fix compiler warnings

libusb/os/windows_usb.c:368:36: warning: no previous prototype for 'get_interface_details_filter' [-Wmissing-prototypes]
libusb/os/windows_usb.c:691:5: warning: no previous prototype for 'get_sub_api' [-Wmissing-prototypes]
libusb/os/windows_usb.c:1179:6: warning: no previous prototype for 'get_api_type' [-Wmissing-prototypes]
Windows: remove useless extern declaration

libusb/os/windows_usb.c:45:13: warning: redundant redeclaration of 'usbi_fd_notification' [-Wredundant-decls]
libusb/libusbi.h:420:6: note: previous declaration of 'usbi_fd_notification' was here
Windows: fix compiler warnings

libusb/os/windows_usb.c:1317:2: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]
libusb/os/windows_usb.c:1317:2: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]
libusb/os/windows_usb.c:1317:2: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]

libusb/os/windows_usb.c:3634:2: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]
libusb/os/windows_usb.c:3634:2: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]
libusb/os/windows_usb.c:3634:2: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]
Windows: fix compiler warning

sizeof() returns a size_t not an int

libusb/os/windows_usb.c:1411:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
libusb/os/windows_usb.c:1416:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]