bancha: add missing rule to build soundstream model

Bypass-Presubmit-Reason: no presubmit flow for this (only seen w/ m simulate and an empty build area)

Change-Id: I1caea1b64b41cce0cb5b240985c429a5cbb13aa7

platforms/bancha/cantrip_builtins.mk

diff --git a/platforms/bancha/cantrip_builtins.mk b/platforms/bancha/cantrip_builtins.mk
index 40b8afa..9bcd810 100644
--- a/platforms/bancha/cantrip_builtins.mk
+++ b/platforms/bancha/cantrip_builtins.mk
@@ -27,3 +27,5 @@
 CANTRIP_MODEL_DEBUG   :=
 
 CANTRIP_SCRIPTS       :=
+
+$(patsubst %.kelvin,%.elf,$(CANTRIP_MODEL_RELEASE)): kelvin_soundstream