Add check-elf-files as a checkbuild prerequisite

This commit adds `check-elf-files` as a `checkbuild` prerequisite so
that prebuilt ELF files can be checked in a `checkbuild` build bot when
`PRODUCT_CHECK_ELF_FILES` is set to `true`.

Bug: 140908693
Test: lunch aosp_walleye-userdebug && make checkbuild
Change-Id: Ifea2f7057eecbf74028e986367f2322ff851f5e4
diff --git a/core/main.mk b/core/main.mk
index b91770b..ba04ece 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -1494,7 +1494,7 @@
 # -------------------------------------------------------------------
 
 .PHONY: checkbuild
-checkbuild: $(modules_to_check) droid_targets
+checkbuild: $(modules_to_check) droid_targets check-elf-files
 
 ifeq (true,$(ANDROID_BUILD_EVERYTHING_BY_DEFAULT))
 droid: checkbuild