uclibc: do not allow the selection of NPTL if snapshot is not selected

In uClibc, NPTL support does not exist in the 0.9.30 branch, that we
are using in Buildroot. It is only available in the uClibc daily
snapshot, extracted from uClibc master branch.

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