util: readline: replace tab indent by four spaces to fix checkpatch errors

Replace tab indent by four spaces to fix errors issued by checkpatch.pl tool
"ERROR: code indent should never use tabs" within "util/readline.c" file.

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20190401024406.10819-3-jbi.octave@gmail.com
Message-Id: <20190401024406.10819-3-jbi.octave@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>