Correct install path for include folder to avoid double nesting

[dantegd]

Similar to rapidsai/cuml#3901.

After #1491 and #rapids-cmake and #1585, now at install time, the cugraph headers are being nested into path/to/env/include/cugraph/cugraph instead of just path/to/env/include/cugraph/. This, as far as I'm aware, is unintentional and unlike the rest of RAPIDS projects (cuDF, RMM and cuML).

cc @trxcllnt @robertmaynard

[codecov-commenter]

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[rlratzel]

Thanks!

[BradReesWork]

@gpucibot merge