examples/nanopb_simple: Modify Makefile
NanoPb submodule could not be updated.
In this case, Makefile of nanopb_simple can't include
nanopb/extra/nanopb.mk
-include $(TOPDIR)/.config
-include $(TOPDIR)/Make.defs
include $(APPDIR)/Make.defs
-include $(TOPDIR)/../external/nanopb/nanopb/extra/nanopb.mk
+-include $(TOPDIR)/../external/nanopb/nanopb/extra/nanopb.mk
APPNAME = nanopb_simple
FUNCNAME = $(APPNAME)_main
