intel/compiler: add cpp_std=c++17 when building tests

Otherwise build fails:

"../src/intel/compiler/brw_private.h:40:4: note:
 ‘std::variant’ is only available from C++17 onwards"

Fixes: 6c194ddd18f ("intel/compiler: Prepare SIMD selection helpers to handle different prog_datas")
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20725>
(cherry picked from commit 53de48f1c43ffc68cf8dbc74b2f5ef00f6b91784)
2 files changed