Cleanup: CMake: Modernize bf_extern_curve_fit_nd dependencies

Pretty straightforward

- Remove any curve_fit_nd paths from INC
- Add a dependency though LIB when missing

context: https://devtalk.blender.org/t/cmake-cleanup/30260

Pull Request: https://projects.blender.org/blender/blender/pulls/110762

extern/curve_fit_nd/CMakeLists.txt

@@ -3,7 +3,7 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
 set(INC
-  .
+  PUBLIC .
 )
 
 set(INC_SYS
@@ -26,3 +26,4 @@ set(LIB
 )
 
 blender_add_lib(extern_curve_fit_nd "${SRC}" "${INC}" "${INC_SYS}" "${LIB}")
+add_library(bf::extern::curve_fit_nd ALIAS extern_curve_fit_nd)