Add missing declaration of override_install_dir

**archiso/mkarchiso**:

In d90184a the unbound variable `override_install_dir` was introduced by accident.

It is required to be set to empty string to provide override functionality for `install_dir`.

Fixes #57

archiso/mkarchiso

@@ -33,6 +33,7 @@ iso_application="${app_name} iso"
 override_iso_application=""
 iso_version=""
 install_dir="${app_name}"
+override_install_dir=""
 arch="$(uname -m)"
 pacman_conf="/etc/pacman.conf"
 override_pacman_conf=""