libtorrent: Add dependency on atomic intrinsics

Libtorrent requires GCC built-in atomic functions which are architecture specific
and may not be implemented.

This fixes:
http://autobuild.buildroot.net/results/51414e22c8ea47c38df20d864526a370c5a895d7/



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