nanopb.mk Makefile rules: fix options dependency (#666)

Files were not being regenerated when .options changed.
diff --git a/extra/nanopb.mk b/extra/nanopb.mk
index 279515f..d60b37d 100644
--- a/extra/nanopb.mk
+++ b/extra/nanopb.mk
@@ -32,6 +32,9 @@
 endif
 
 # Rule for building .pb.c and .pb.h
-%.pb.c %.pb.h: %.proto $(wildcard %.options)
+%.pb.c %.pb.h: %.proto %.options
+	$(PROTOC) $(PROTOC_OPTS) --nanopb_out=. $<
+
+%.pb.c %.pb.h: %.proto
 	$(PROTOC) $(PROTOC_OPTS) --nanopb_out=. $<