Add delta_generator module dependencies.

delta_generator requires bsdiff and imgdiff installed in the host to
run so we need to depend on those when building delta_generator.

Bug: None
TEST=`mmma system/update_engine` builds bsdiff and imgdiff.

Change-Id: Id8cb2c0b86780b12cfc4010439c0f3be22eb20a7
diff --git a/Android.mk b/Android.mk
index 77bb420..8084b71 100644
--- a/Android.mk
+++ b/Android.mk
@@ -644,6 +644,9 @@
 # Build for the host.
 include $(CLEAR_VARS)
 LOCAL_MODULE := delta_generator
+LOCAL_REQUIRED_MODULES := \
+    bsdiff \
+    imgdiff
 LOCAL_MODULE_CLASS := EXECUTABLES
 LOCAL_CPP_EXTENSION := .cc
 LOCAL_CLANG := true