Fix pluck_impl() signature

[DavisVaughan]

Noted on CRAN's LTO build https://www.stats.ox.ac.uk/pub/bdr/LTO/purrr.out

pluck_impl() doesn't actually use error_call at the C level. I think this was just a holdover from when we were experimenting with passing that argument through to C, but then decided on another way.

init.c:12:13: warning: type of ‘pluck_impl’ does not match original declaration [-Wlto-type-mismatch]
   12 | extern SEXP pluck_impl(SEXP, SEXP, SEXP, SEXP, SEXP);
      |             ^
pluck.c:203:6: note: type mismatch in parameter 5
  203 | SEXP pluck_impl(SEXP x, SEXP index, SEXP missing, SEXP strict_arg) {
      |      ^
pluck.c:203:6: note: type ‘void’ should match type ‘struct SEXPREC *’
pluck.c:203:6: note: ‘pluck_impl’ was previously declared here

Gabor said he has fixed the other cli -Wstrict-prototypes issue