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.

I'm sure we can come up with a better solution, but this unbreaks builds.

Closes: #11178 Fixes: 53629b0a ("rusticl: make use of new output_inline_wrapper meson.rust.bindgen feature")