scripts: generate_rust_analyzer: add missing include_dirs

Commit 8c4555cc ("scripts: add `generate_rust_analyzer.py`")
specified OBJTREE for the bindings crate, and `source.include_dirs` for
the kernel crate, likely in an attempt to support out-of-source builds
for those crates where the generated files reside in `objtree` rather
than `srctree`. This was insufficient because both bits of configuration
are required for each crate; the result is that rust-analyzer is unable
to resolve generated files for either crate in an out-of-source build.

  [ Originally we were not using `OBJTREE` in the `kernel` crate, but
    we did pass the variable anyway, so conceptually it could have been
    there since then.

    Regarding `include_dirs`, it started in `kernel` before being in
    mainline because we included the bindings directly there (i.e.
    there was no `bindings` crate). However, when that crate got
    created, we moved the `OBJTREE` there but not the `include_dirs`.
    Nowadays, though, we happen to need the `include_dirs` also in
    the `kernel` crate for `generated_arch_static_branch_asm.rs` which
    was not there back then -- Tamir confirms it is indeed required
    for that reason. - Miguel ]

Add the missing bits to improve the developer experience.

Fixes: 8c4555cc ("scripts: add `generate_rust_analyzer.py`")
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250210-rust-analyzer-bindings-include-v2-1-23dff845edc3@gmail.com


[ Slightly reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>