gtest: install libgtest_main to staging

gtest provides a separate libgtest_main library with a default main()
implementation for tests with basic needs.
This separate library isn't being installed by buildroot. This patch
adds the installation of this library to staging during the install of
gtest.

Signed-off-by: Erico Nunes <erico.nunes@datacom.ind.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>