Updates for -Wstrict-prototypes

[DavisVaughan]

As requested by CRAN in the email about slider:

* 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]

Not all yours: can you pls liaise with the vctrs maintainer?

[DavisVaughan]

When this PR is used in combination with an updated rlang C library (after r-lib/rlang#1508 is merged), vctrs is now quiet