rusticl/meson: add build root dir to the include dirs of rusticl_c

The static inline wrapper includes the header file relatively from where
`bindgen` gets executed, or so it seems.

And because meson doesn't allow us to add absolute paths, fs.relative_to
needs to be used. I'm sure we can come up with a better solution, but this
unbreaks builds.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/11178
Fixes: 53629b0a2d ("rusticl: make use of new `output_inline_wrapper` meson.rust.bindgen feature")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29275>

src/gallium/frontends/rusticl/meson.build

@@ -18,6 +18,7 @@
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 
+fs = import('fs')
 rust = import('rust')
 
 libmesa_rust_util_files = files(
@@ -348,6 +349,7 @@ rusticl_c = static_library(
   ],
   gnu_symbol_visibility : 'hidden',
   include_directories : [
+    fs.relative_to(meson.project_build_root(), meson.current_source_dir()),
     inc_gallium,
     inc_gallium_aux,
     inc_include,