Adding adb_debug.prop into generic_ramdisk.mk

After commit I59cd149e0021211b8fd59c44b93bbf18dc8637bf, init_first_stage
is no longer installed to the ramdisk when BOARD_USES_RECOVERY_AS_BOOT
is True (see b/190974433 for details and init/Android.mk in that commit).

adb_debug.prop, which is needed for boot-debug.img,
boot-test-harness.img, etc.,will be absent when
BOARD_USES_RECOVERY_AS_BOOT is true, because adb_debug.prop is
only required by the init_first_stage.

Adding adb_debug.prop into generic_ramdisk.mk to include it
unconditionally.

Bug: 192432810
Test: flash boot-debug.img on a user build, check `adb root` works
Change-Id: I234b3430af1b8b09aeb46aef928ca8e16ad452cc