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