Update FindDPCPP to find new include path

[Rbiessy]

• Update FindDPCPP.cmake to be able to find the new path to the include path
• Removed branch if(NOT DPCPP_FOUND) return() as find_package_handle_standard_args will already fail with an error if either required vars cannot be found

Tested with modules compiler/2023.0.0 and compiler/2024.0.2.

Checklist

Tick if relevant:

• [N/A] New files have a copyright
• [N/A] New headers have an include guards
• [N/A] API is documented with Doxygen
• [N/A] New functionalities are tested
• Tests pass locally
• [N/A] Files are clang-formatted

[hjabird]

I had a go with clang++ built from intel/llvm and that also seemed to work.