Add explicit "# key" directive to legacy snippet collection before deprecating filenames-as-triggers

snippets/c-mode/printf

@@ -1,6 +1,7 @@
 # -*- mode: snippet -*-
 # name : printf 
 # contributor : joaotavora
+# key: printf
 # --
 printf ("${1:%s}\\n"${1:$(if (string-match "%" text) "," "\);")
 }$2${1:$(if (string-match "%" text) "\);" "")}