In C function declarations, use void

[wch]

This fixes some warnings on R 4.3.0:

checking whether package ‘later’ can be installed ... WARNING
Found the following significant warnings:
  init.c:12:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
  init.c:18:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
  init.c:24:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
  init.c:27:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
  init.c:51:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
See https://www.r-project.org/nosvn/R.check/r-patched-linux-x86_64/later-00install.html for details.