boost: add fiber module

This module requires NPTL. Without support for the module, it is built
unconditionally, which was causing the following build errors:
http://autobuild.buildroot.net/results/029/0298038fc126d15733d81c54e0bb7cb00be48b92/build-end.log
http://autobuild.buildroot.net/results/6f3/6f3a218c47204e431100799482a3ed0ec159fa15/build-end.log
http://autobuild.buildroot.net/results/63e/63e5569a90d3ace97cb6102509cbd04aeab6f5f7/build-end.log



Signed-off-by: Adam Duskett <aduskett@gmail.com>
[Arnout: add empty line in Config.in, reword commit message]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>