libs/icu: fixed hardcoded path in icu-config

Summary:
Current recipe generates bin/icu-config script that contains a hardcoded path inside:

default_prefix="/home/appimage/Craft/BinaryCache/linux-64-gcc"
if [ "x${prefix}" = "x" ]; then prefix="$default_prefix"; fi

Using icu-config without specifiying prefix fails. This patch solves that by replacing those lines with:

default_prefix="$KDEROOT"
if [ "x${prefix}" = "x" ]; then prefix="$default_prefix"; fi

Reviewers: Craft, vonreth

Reviewed By: Craft, vonreth

Differential Revision: https://phabricator.kde.org/D25687