Refactor: BKE: Move icon header to C++

Was trying to use a C++ type (`blender::Span`) in the header but
couldn't. I would rather not use a less safe raw pointer to a buffer.

Pull Request: https://projects.blender.org/blender/blender/pulls/148214

source/blender/python/intern/bpy_app_icons.cc

@@ -12,7 +12,7 @@
 
 #include "MEM_guardedalloc.h"
 
-#include "BKE_icons.h"
+#include "BKE_icons.hh"
 
 #include "../generic/py_capi_utils.hh"
 #include "../generic/python_compat.hh" /* IWYU pragma: keep. */