Add a static c_function to be strictly local to this unit

Property definitions

nitc $ CCompilationUnit :: add_local_function
	# Add a `static` `c_function` to be strictly local to this unit
	fun add_local_function(c_function: CFunction)
	do
		body_decl.add "static {c_function.signature};\n"
		body_impl.add "\n"
		body_impl.add c_function.to_writer
	end
src/c_tools.nit:50,2--56,4