buildroot

privoxy: needs thread support

Even though privoxy has some conditional code to handle configurations
without threads, it is not properly handled everywhere, leading to a
build failure. Since this package is unlikely to be used in
non-threaded environment, just disable it for non-threaded
configurations.

Fixes:

  http://autobuild.buildroot.net/results/fae7a41d110ed6c4d62c77873229d069d8b65986

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>