Add the missing dependency on payload generation script.

For A/B OTAs, ota_from_target_files.py calls brillo_update_payload
script to generate the payload entry in an A/B OTA package.

Bug: 37443093
Test: lunch aosp_marlin-userdebug && rm
      out/host/linux-x86/bin/brillo_update_payload && m otapackage
Change-Id: I3845786b6c934c709f32e12a63cd6ea1f303d380
diff --git a/core/config.mk b/core/config.mk
index 884be1e..583909d 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -634,6 +634,7 @@
 FUTILITY := $(HOST_OUT_EXECUTABLES)/futility-host
 VBOOT_SIGNER := prebuilts/misc/scripts/vboot_signer/vboot_signer.sh
 FEC := $(HOST_OUT_EXECUTABLES)/fec
+BRILLO_UPDATE_PAYLOAD := $(HOST_OUT_EXECUTABLES)/brillo_update_payload
 
 DEXDUMP := $(HOST_OUT_EXECUTABLES)/dexdump2$(BUILD_EXECUTABLE_SUFFIX)
 PROFMAN := $(HOST_OUT_EXECUTABLES)/profman