a68: fix wrapping C functions returning void [PR algol68/124322]

This patch fixes a68_wrap_formal_proc_hole so it doesn't assume that
wrapped C functions returning void return Algol 68 void values, which
are empty records.

Tested in i686-linux-gnu and x86_64-linux-gnu.

Signed-off-by: Jose E. Marchesi <jemarch@gnu.org>

gcc/algol68/ChangeLog

	PR algol68/124322
	* a68-low-holes.cc (a68_wrap_formal_proc_hole): Wrap functions
	returning void properly.