CMake: only create uninstall target when eigen is top level

Reference issue

#1892 (closed)

What does this implement/fix?

When building eigen with other libraries using CMake's FetchContent, the uninstall target might clash between these libraries. User might encounter error similar to the following (the other library here is raylib)

CMake Error at build/_deps/eigen3-src/CMakeLists.txt:648 (add_custom_target):
  add_custom_target cannot create target "uninstall" because another target
  with the same name already exists.  The existing target is a custom target
  created in source directory "/build/_deps/raylib-src".  See documentation for 
  policy CMP0002 for more details.

The solution is fairly straight-foward: only build uninstall target when building eigen by itself by using PROJECT_IS_TOP_LEVEL.

Additional information