[Documentation Fix] Building mkl_interface example is failing as it can't find example_helper.hpp

[sreerajkksd]

Building the C++ example in https://github.com/IntelPython/example-portable-data-parallel-extensions/blob/main/steps/mkl_interface/building.md#building-a-c-example-that-uses-the-library fails with

> icpx -fsycl getrs_usm.cpp -fsycl-targets=nvptx64-nvidia-cuda,spir64-unknown-unknown -I${MKL_INTERFACE_ROOT}/include -L${MKL_INTERFACE_ROOT}/lib -lonemkl -Wl,-rpath,${MKL_INTERFACE_ROOT}/lib -o run -I -I${MKL_INTERFACE_ROOT}/../examples/include
getrs_usm.cpp:48:10: fatal error: 'example_helper.hpp' file not found
   48 | #include "example_helper.hpp"
      |          ^~~~~~~~~~~~~~~~~~~~
1 error generated.

I added -I${MKL_INTERFACE_ROOT}/../examples/include to the icpx command and that seems to have worked.