MAINTAINERS: add spdxexclude to LICENSES and SPDX stuff

Commit 0509b270

 ("scripts/spdxcheck: Put excluded files and directories
into a separate file") moved excluded files to the new file
scripts/spdxexclude, but did not adjust the MAINTAINERS section. The file
scripts/spdxexclude clearly belongs to LICENSES and SPDX stuff.

Add a corresponding file entry.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20220825152029.12660-1-lukas.bulwahn@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>