Add the "all" target to the list of PHONY targets

Looks like this one has been missed.

Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

Makefile

@@ -24,4 +24,4 @@ json-schema.tar.gz: $(OUT)
 clean:
 	rm -f $(OUT) json-schema.tar.gz
 
-.PHONY: clean
+.PHONY: clean all