cmake: llext: Fix llext incremental build

Before this commit, building llext packages with a `POST_BUILD` step added
via `add_llext_command()` only worked for from-scratch builds (tested with
Ninja).

When the client CMake script called `add_llext_command()` to add a custom
`POST_BUILD` step, `has_post_build_cmds` was set to 1, and a `true` (do
nothing) command was executed instead of `copy` for the `llext_pkg_input`
target. Consequently, the timestamp of `llext_pkg_input` was not updated.
Before executing `llext_proc_target`, Ninja seems to check the timestamp
of `llext_pkg_input`. When the custom command added by
`add_llext_command()` generated a new `llext_pkg_input`, Ninja was unaware
that `llext_pkg_input` had been updated, and the dependent
`llext_pkg_output` was not executed. Building only worked if
`llext_pkg_input` was missing, i.e., only for from-scratch builds.

The fix replaces dependencies so the command for `llext_pkg_input` is
executed after `llext_proc_target` (after `llext_proc_target` may generate
a new `llext_pkg_input`).

Signed-off-by: Serhiy Katsyuba <serhiy.katsyuba@intel.com>