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]