libuv: needs NPTL

libuv: needs NPTL

Fixes:
http://autobuild.buildroot.net/results/b81/b81583dea1bdf3777b2f2736c84fef90aa10f30b/
http://autobuild.buildroot.net/results/93f/93f1c4dcaa9a20ec62c547b839cd334ed1b5fbe4/
http://autobuild.buildroot.net/results/030/030ef0a9bd51a2c52d6026e0c1e383a9bdae3c4f/
http://autobuild.buildroot.net/results/fff/fff2098f707d9827ec5e1fd38fe742dd2b695ead/

libuv uses pthread_barrier_* functions, which aren't available with
linuxthreads.  Notice that libuv contains a local prototype for these
functions, so libuv is able to build and the error only triggers when
applications try to link against it.

Also propagate this dependency to the reverse deps of libuv.

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