Input: adxl34x - make platform_data include self contained

Since it suggests to use defines from input.h (ABS_X, ...), also include
the file to make them available.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>