tcl: needs threads

Fixes http://autobuild.buildroot.net/results/486/48696710a270ac69d6d62654779f164075cb3451/



Tcl itself has a --disable-threads configure flag, but it unconditially
configures its packages with --enable-threads, causing build failure.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>