package/glog: fix build without threads

Fix the following build failure without threads raised since bump to
version 0.6.0 in commit 017dbc77 and
https://github.com/google/glog/commit/4a4331f2f2de4c35ce72badf0bdc604c04eebb26:

CMake Error at /home/giuliobenetti/autobuild/run/instance-1/output-1/build/glog-0.6.0/CMakeFiles/CMakeTmp/CMakeLists.txt:18 (add_executable):
  Target "cmTC_ed950" links to target "Threads::Threads" but the target was
  not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?

Fixes:
 - http://autobuild.buildroot.org/results/bf0846a51da69169286c7af38089d204d3d242d1



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>