Deal with (void) function signature warnings

[DavisVaughan]

* checking whether package ‘slider’ can be installed ... [5s/5s] WARNING
Found the following significant warnings:
  /home/Hornik/lib/R/Library/4.3/x86_64-linux-gnu/vctrs/include/vctrs.h:12:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
  init.c:69:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
  init.c:72:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
  slider-vctrs-private.c:12:6: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
  /home/Hornik/lib/R/Library/4.3/x86_64-linux-gnu/vctrs/include/vctrs.c:7:6: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
  slider-vctrs-public.c:3:6: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
  summary-core.h:9:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
  summary-core.h:10:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]