GRMLBASE/45-grub-images: adapt script for renamed raid.mod file

Up until (and including) grub version 1.99-27.1 it's called
raid.mod, starting with grub version 2.00-14 the raid.mod
file no longer exists.

Fixes:

| =====   shell: GRMLBASE/45-grub-images   =====
| grub-mkimage: error: cannot stat `/usr/lib/grub/i386-pc/raid.mod': No such file or directory.
| GRMLBASE/45-grub-images FAILED with exit code 1.

Thanks: Jordan Uggla for the feedback