INPUT: sgi_btns: Add license specification

The SGI Volume Button interface driver uses GPL-only symbols
platform_driver_unregister and platform_driver_register, but
lacks license specification. Thus, when compiled as a module,
this driver cannot be installed. This patch fixes this by
adding the MODULE_LICENSE() specification.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>