package/gtest: needs gcc >= 4.9

package/gtest: needs gcc >= 4.9

gtest unconditionally uses is_trivially_copy_constructible since
version 1.11.0 and
https://github.com/google/googletest/commit/c13c27a513ecd1cbf5700a45fe590e85e8ae6770

So add a dependency on gcc >= 4.9 to avoid a build failure since commit
9dfbbbb4105c47602da048c9bb9499fb8862e768

Fixes:
 - http://autobuild.buildroot.org/results/3d23d7c2c340c23e468890e4df73a9b3b142ecf6

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