libeio: needs threads

Fixes:
http://autobuild.buildroot.net/results/278/27854bf8425d64dbb475653161bbf1f386173366/



libeio only reverse dependency (enlightenment) depends on
BR2_TOOLCHAIN_HAS_THREADS already.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>