brillo_update_payload should require simg2img.

It's using simg2img to convert sparse images for delta_generator, this
dependency wasn't reflected in Android.mk

Test: mma
Change-Id: I7c67183bee2db756062196cfe5b4676e2a5832b4
diff --git a/Android.mk b/Android.mk
index 528aa76..ff12e59 100644
--- a/Android.mk
+++ b/Android.mk
@@ -1007,6 +1007,7 @@
 LOCAL_MODULE_TAGS := optional
 LOCAL_REQUIRED_MODULES := \
     delta_generator \
-    shflags
+    shflags \
+    simg2img
 include $(BUILD_PREBUILT)
 endif  # HOST_OS == linux