Mark more targets as PHONY

So that we can start restricting non-phony targets to $OUT_DIR and
$DIST_DIR.

Test: add --writable=out/, see fewer warnings
Change-Id: I411fe4af732b1bce35d4a4c1e2a47b5f8a15e6b8
diff --git a/core/product-graph.mk b/core/product-graph.mk
index 51985df..696aabf 100644
--- a/core/product-graph.mk
+++ b/core/product-graph.mk
@@ -145,3 +145,4 @@
 	dot -Tsvg -Nshape=box -o $@ $<
 
 product-graph: $(products_pdf) $(products_svg)
+.PHONY: product-graph