cmake: Remove warnings for gtest
cmake: Remove warnings for gtest

Add flag SYSTEM to `include_directories` for gtest
This omits warnings generated in gtest code-base

Reference: https://cmake.org/cmake/help/latest/command/include_directories.html

JohanMabille/xtensor,xtensor-stack/xtensor,QuantStack/xtensor,xtensor-stack/xtensor,QuantStack/xtensor,JohanMabille/xtensor