Correct install path for include folder to avoid double nesting (#1630)

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

Authors:
  - Dante Gama Dessavre (https://github.com/dantegd)

Approvers:
  - Chuck Hastings (https://github.com/ChuckHastings)
  - Robert Maynard (https://github.com/robertmaynard)
  - Seunghwa Kang (https://github.com/seunghwak)
  - Paul Taylor (https://github.com/trxcllnt)
  - Rick Ratzel (https://github.com/rlratzel)

URL: #1630

cpp/CMakeLists.txt

@@ -335,7 +335,7 @@ install(TARGETS cugraph
         EXPORT cugraph-exports)
 
 install(DIRECTORY include/
-        DESTINATION include/cugraph)
+        DESTINATION include)
 
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/include/cugraph/version_config.hpp
         DESTINATION include/cugraph)