chore: add .gitignore

[e-kwsm]

No description provided.

[BiplabRaut]

Hi @e-kwsm,
Thank you for submitting this patch.

As a standard practice, it will be good to run the basic sanity, and functional tests.
Here are the recommended steps to run a variety of tests :

Configure the library using a combination of configure options like --enable-sse2 --enable-avx --enable-avx2 --enable-avx512 --enable-mpi --enable-openmp --enable-shared --enable-amd-opt --enable-amd-mpifft --enable-dynamic-dispatcher --enable-amd-mpifft --enable-amd-fast-planner --enable-amd-top-n-planner --enable-amd-app-opt --enable-amd-trans
Generate a different build of the library based on the configure command
Run "make small check" , "make check" "make big check"

Please share/attach the results with this PR itself.
Thanks

[e-kwsm]

This does not affect any FFTW functionalities.