Add missing vendor_kernel_ramdisk dependency to target-files.zip

The rule that builds target-files.zip has no dependency on the
vendor_kernel_ramdisk image, which means the files in that image
may not be in place when target-filse.zip is built, causing a
build error:
mkbootfs: cannot open directory '/mnt/disks/build-disk/src/googleplex-android/aosp-main-future-without-vendor/out/target/product/cheetah/obj/PACKAGING/target_files_intermediates/aosp_cheetah-target_files/VENDOR_KERNEL_BOOT/RAMDISK': No such file or directory

Add the missing dependency.

Bug: 365887100
Test: none
Flag: EXEMPT bugfix
Change-Id: I5cdec9bbb35970e4d5e27a6b10ad118ce11835a9