Depend on (bs|img)diff for target-files-package

add_img_to_target_files may use these to create the recovery patch, so
add a dependency. They were previously a transitive dependency through
the system image to the standalone recovery patch creation.

Bug: 36575896
Test: rm out/host/linux-x86/bin/imgdiff; m -j target-files-package
Change-Id: I2a43220e94c09393e88e1d9950032f5665a0d2ce
diff --git a/core/Makefile b/core/Makefile
index 3a0d343..74ffc96 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1955,6 +1955,8 @@
 		$(APKCERTS_FILE) \
 		$(SOONG_ZIP) \
 		$(HOST_OUT_EXECUTABLES)/fs_config \
+		$(HOST_OUT_EXECUTABLES)/imgdiff \
+		$(HOST_OUT_EXECUTABLES)/bsdiff \
 		$(BUILD_IMAGE_SRCS) \
 		| $(ACP)
 	@echo "Package target files: $@"