dma-mapping: add missing `inline` for `dma_free_attrs`

Under an UML build for an upcoming series [1], I got `-Wstatic-in-inline`
for `dma_free_attrs`:

      BINDGEN rust/bindings/bindings_generated.rs - due to target missing
    In file included from rust/helpers/helpers.c:59:
    rust/helpers/dma.c:17:2: warning: static function 'dma_free_attrs' is used in an inline function with external linkage [-Wstatic-in-inline]
       17 |         dma_free_attrs(dev, size, cpu_addr, dma_handle, attrs);
          |         ^
    rust/helpers/dma.c:12:1: note: use 'static' to give inline function 'rust_helper_dma_free_attrs' internal linkage
       12 | __rust_helper void rust_helper_dma_free_attrs(struct device *dev, size_t size,
          | ^
          | static

The issue is that `dma_free_attrs` was not marked `inline` when it was
introduced alongside the rest of the stubs.

Thus mark it.

Fixes: ed6ccf10f24b ("dma-mapping: properly stub out the DMA API for !CONFIG_HAS_DMA")
Closes: https://lore.kernel.org/rust-for-linux/20260322194616.89847-1-ojeda@kernel.org/ [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20260325015548.70912-1-ojeda@kernel.org