core/Makefile: system_dlkm: Fix non avb dep typo

$(MKFSEROFS) -> $(MKEROFS) in target dependency
for INSTALLED_SYSTEM_DLKM_IMAGE_TARGET in case
when AVB is not enabled.

Bug: 200082547
Test: TH
Test: atest vts_system_dlkm_partition_test
Fixes: 1065523a ("Add support to build system_dlkm.img")
Signed-off-by: Ramji Jiyani <ramjiyani@google.com>
Change-Id: I03a4463b1fa01b95e706d623aa8a3888e37488d6