libdvdnav: requires thread support in toolchain

libdvdnav requires thread support. The only package that depends on
libdvdnav is mplayer, and it is an optional dependency, only activated
when libdvdnav is enabled. So we don't have to push this thread
support dependency to any other package.

Fixes:

  http://autobuild.buildroot.org/results/54d6a737ae805ef1dbf77e5d11b4dd5366873ec0/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>