Add license header to display_utils.rs and pretty_print.rs

[alamb]

• Part of Release sqlparser-rs version 0.57.0 around 2024-06-15 #1837

When I tried to make a release caiddate for 0.57.0 I got some errors about files not having the Apache License:

Running rat license checker on /Users/andrewlamb/Software/sqlparser-rs/dev/dist/apache-datafusion-sqlparser-rs-0.57.0-rc1/apache-datafusion-sqlparser-rs-0.57.0.tar.gz
NOT APPROVED: src/display_utils.rs (apache-datafusion-sqlparser-rs-0.57.0/src/display_utils.rs): false
NOT APPROVED: tests/pretty_print.rs (apache-datafusion-sqlparser-rs-0.57.0/tests/pretty_print.rs): false

This PR adds the missing licenses

I will make another PR to add a license checking CI test

[iffyio]

LGTM!

[alamb]

Thansk @iffyio -- I am now preparing the release candidate