Input: include export.h in modules using EXPORT_SYMBOL*()

Input: include export.h in modules using EXPORT_SYMBOL*()

A number of modules in the input subsystem use EXPORT_SYMBOL() and
friends without explicitly including the corresponding header
<linux/export.h>. While the build currently succeeds due to this header
being pulled in transitively, this is not guaranteed to be the case in
the future.

Let's add the explicit include to make the dependencies clear and
prevent future build breakage.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>