staging: r8188eu: remove unused define

Remove the _IOCTL_LINUX_C_ define, which is not used.

Acked-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211205151251.6861-10-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>