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