Fix cmake installation directory

Summary:
We moved all header files into a subdirectory `include/sparta/`, but didn't change the CMakeLists.txt
This currently leads to installing header files under `include/sparta/sparta`.
Let's fix that.

Reviewed By: arnaudvenet

Differential Revision: D52070290

fbshipit-source-id: 0c6fb4f08beb688b9e47b615fe6b2aaea2925191

CMakeLists.txt

@@ -44,7 +44,7 @@ install(TARGETS sparta EXPORT sparta_target
     ARCHIVE  DESTINATION ${CMAKE_INSTALL_LIBDIR}
     LIBRARY  DESTINATION ${CMAKE_INSTALL_LIBDIR}
     RUNTIME  DESTINATION ${CMAKE_INSTALL_BINDIR})  # This is for Windows
-install(DIRECTORY include/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/sparta)
+install(DIRECTORY include/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 
 install(EXPORT sparta_target DESTINATION cmake)