protobuf: link with libatomic when needed

The protobuf library uses atomic intrinsics, so we need to link
against libatomic.

Fixes the build of protobuf on Sparc:

  http://autobuild.buildroot.net/results/f3d76eaebd529a61bce849e355182c60f233ed06/



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