rocclr: fix build (no OpenCL library is required)
rocclr: fix build (no OpenCL library is required)

rocclr builds fail since CMake was upgraded to 3.18. This version
introduces the REQUIRED option of find_library. However rocclr only
needs OpenCL headers. This change makes finding an OpenCL library
optional again.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs