tests/headers/CMakeLists.txt: Remove extra "/" in include paths

It shouldn't be needed. Absolute paths won't be made more absolute by an additional / at the start, relative paths would be turned into nonsense.

Downstream in Nixpkgs, this causes an issue with out CC wrapper where all of the includes are considered impure & stripped from the compiler invocations: https://github.com/NixOS/nixpkgs/issues/8676