Sandbox c2hal_test_genc++[_headers]

These genrules already work with sandboxing, I'm not sure why they
were added to the list.

Bug: 307824623
Test: ./build/soong/tests/genrule_sandbox_test.py c2hal_test_genc++_headers c2hal_test_genc++
Change-Id: I697c9cff1db0bf71b3608684fde73535a72f71b2