privoxy: fix preinst/postinst script indentation

privoxy: fix preinst/postinst script indentation

Fix the indentation of the preinst/postinst scripts for the privoxy
package.

Because these scripts didn't start with `#!/bin/sh`
(they instead started with the TAB character), `/bin/sh` was not used
to start them.

On x86_64 and i386_pentium-mmx, this seems to be fine, but on
arm_cortex-a15_neon-vfpv4 and aarch64_cortex-a53, running these
scripts fails with a:

```
Installing privoxy (3.0.33-3) to root...
Collected errors:
 * pkg_run_script: package "privoxy" preinst script returned status 1.
 * preinst_configure: Aborting installation of privoxy.
 * opkg_install_cmd: Cannot install package privoxy.
```

Reported-by: Marius Dinu <m95d+git@psihoexpert.ro>
Signed-off-by: Alois Klink <alois@aloisklink.com>
(cherry picked from commit 6703d7623097b591ea08c12e66351eed884e1005)