Cleanup: CMake: Modernize bf_imbuf dependencies

Pretty straightforward

- Remove any bf_imbuf 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/115425

source/blender/python/mathutils/CMakeLists.txt

@@ -6,7 +6,6 @@ set(INC
   .
   ../../blenkernel
   ../../bmesh
-  ../../imbuf
 )
 
 set(INC_SYS
@@ -43,7 +42,7 @@ set(LIB
   PRIVATE bf::blenlib
   PRIVATE bf::depsgraph
   PRIVATE bf::dna
-  bf_imbuf
+  PRIVATE bf::imbuf
   PRIVATE bf::intern::guardedalloc
   bf_python_ext