input: Add header file to the longpress example

Add the header file required for the example to work. Users get the
following error due to unresolved INPUT_BTN_* values otherwise:

parse error: expected number or parenthesized expression

Signed-off-by: Amit Kucheria <amitk@kernel.org>
Signed-off-by: Amit Kucheria <amit@mbedrock.com>