Move comment to its intended location

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/Makefile b/Makefile
index b324b9a..885948c 100644
--- a/Makefile
+++ b/Makefile
@@ -54,6 +54,8 @@
 ifdef GEN_FILES
 gen_file_dep =
 else
+# Order-only dependency: generate the target if it's absent, but don't
+# re-generate it if it's present but older than its dependencies.
 gen_file_dep = |
 endif