cmake: adjust dependencies added for libuv

Commit 1ffcf364 updated cmake to 3.7.0,
which requires selecting the libuv package. At the time, the libuv
package only depended on BR2_TOOLCHAIN_HAS_THREADS. However, later on,
it was changed in master to depend on BR2_TOOLCHAIN_HAS_THREADS_NPTL, a
change which was not taken into account in the cmake 3.7.0 bump that was
merged in the next branch.

Due to this, builds of cmake is attempted on architectures that don't
provide NPTL thread support, causing a build failure. This commit fixes
that by adjusting the dependency.

Fixes:

  http://autobuild.buildroot.net/results/16a5e1cbb57c0124537c4f3dc0807ba1eaa975ec/



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