srt: Fix wrong `srt.pc` include path. Fixes #70872.

Fix copied from:

    https://github.com/NixOS/nixpkgs/blob/1230fc8674439df5a90fe4c1ea740256ef9906e4/pkgs/development/compilers/hhvm/default.nix#L41-L44



Signed-off-by: Niklas Hambüchen <mail@nh2.me>