Fix aosp_arm64 build error when `make ramdisk_debug`

Some targets have ramdisk.img but no boot.img, howerver,
ramdisk-debug.img only depends on boot.img. Fix this by making
ramdisk-debug.img depends on ramdisk.img.

Bug: 126493225
Test: make ramdisk_debug
Change-Id: I65120a3b3372712fafc26442354ee031eede0bd3
Merged-In: I65120a3b3372712fafc26442354ee031eede0bd3
(cherry picked from commit c3e8cc9b)