Allow rocksdb library to be usable with CMake's `FetchContent` API (#11575)

Summary:
This adds proper support for using rocksdb with FetchContent, without this PR the user must include the following with their own `CMakeLists.txt` file:
```cmake
include_directories(./build/_deps/rocksdb-src/include)
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11575

Reviewed By: jowlyzhang

Differential Revision: D47163520

Pulled By: ajkr

fbshipit-source-id: a202dcf435ecc9dd8d51c88f90e98c04814721ca

CMakeLists.txt

@@ -1592,3 +1592,6 @@ option(WITH_BENCHMARK "build benchmark tests" OFF)
 if(WITH_BENCHMARK)
   add_subdirectory(${PROJECT_SOURCE_DIR}/microbench/)
 endif()
+
+target_include_directories(${PROJECT_NAME} PUBLIC
+  $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>)