google-breakpad: needs thread support

When google-breakpad was enabled in uClibc configurations, the
dependency on threads was forgotten.

Fixes:

  http://autobuild.buildroot.net/results/6e6bc845314100f2deb8ed06d2a6373ccf715ffe/



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