scripts: fix installed-size calculation

Previously the script would calculate the size of the compressed archive
which isn't the size installed in the overlayfs.

This commit uses zcat in combination with wc to calculate the
umcompressed size.

Signed-off-by: Paul Spooren <mail@aparcar.org>

scripts/ipkg-build

@@ -181,7 +181,7 @@ for file_mode in $file_modes; do
 done
 $TAR -X "$tmp_dir"/tarX --format=gnu --numeric-owner --sort=name -cpf - --mtime="$TIMESTAMP" . | gzip -n - > "$tmp_dir"/data.tar.gz
 
-installed_size=$(stat -c "%s" "$tmp_dir"/data.tar.gz)
+installed_size=$(zcat < "$tmp_dir"/data.tar.gz | wc -c)
 sed -i -e "s/^Installed-Size: .*/Installed-Size: $installed_size/" \
 	"$pkg_dir"/$CONTROL/control