makekeys: Fix build/target word size mismatch when cross-compiling

makekeys: Fix build/target word size mismatch when cross-compiling

This matches commit 24283d40b1e4314c6647dda49d2a159833341a8b from
libX11:

    Since makekeys is built using build environment's compiler and
    runs natively, we have to make sure that the size of the
    Signature type is the same on both the native environment
    and the target, otherwise we get mismatches upon running X,
    and some LSB test failures (xts5).

    Use an unsigned 32-bit integer on all platforms.

    Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
    Reviewed-by: Daniel Stone <daniel@fooishbar.org>
Signed-off-by: Ran Benita <ran234@gmail.com>