r-leafem: add optdepends

[bshor]

Fix the r-leafem metadata failure exposed after r-geojsonsf rebuilt
successfully.

Add the non-base CRAN Suggests packages to optdepends and bump pkgrel to
trigger a rebuild.

Verification:

• git diff --check
• git diff --check --cached
• bash -n BioArchLinux/r-leafem/PKGBUILD
• makepkg --verifysource
• makepkg -Cfd