whisper : add makefile target to update ggml submodule

This commit adds a recipe/target to the makefile to update the ggml
submodule.

The motivation for this is that it is not always clear how to update and
what if any flags are needed to update the submodule.

Makefile

@@ -56,3 +56,8 @@ tiny.en tiny base.en base small.en small medium.en medium large-v1 large-v2 larg
 		./build/bin/whisper-cli -m models/ggml-$@.bin -f $$f ; \
 		echo "" ; \
 	done
+
+.PHONY: update-ggml
+update-ggml:
+	echo "Updating ggml submodule..."
+	git submodule update --remote ggml