modules: nanopb: Add custom target for generated header files

Nanopb generates header files that need to be available before being
included. This isn't an issue for a target where the files were added
with zephyr_nanopb_sources. But if another target wants to include
these generated files we need a cmake dependency chain.

The nanopb_generated_headers custom target been added for this purpose.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>