minimal/Makefile: Explicitly include lib/utils/printf.c in build.

The bare-metal port needs it and it's no longer included by default since
the Makefile now uses $(PY_CORE_O).

ports/minimal/Makefile

@@ -41,6 +41,7 @@ LIBS =
 SRC_C = \
 	main.c \
 	uart_core.c \
+	lib/utils/printf.c \
 	lib/utils/stdout_helpers.c \
 	lib/utils/pyexec.c \
 	lib/libc/string0.c \