add importFrom grDevices cm

[rsbivand]

I see that work on the plot key.width issue is ongoing. In looking at GDAL 3.8.1RC3 (from last night), I noticed:

* checking R code for possible problems ... NOTE
.image_scale_factor : <anonymous>: no visible global function
  definition for ‘cm’
kw_dflt: no visible global function definition for ‘cm’
Undefined global functions or variables:
  cm
Consider adding
  importFrom("grDevices", "cm")
to your NAMESPACE file.

so added to R/init.R and ran devtools::document, hence this PR.

[edzer]

Ongoing indeed, hope to continue / finish Thu later & Friday