silence compiler warning in droppables()

Reported by entrez as a github issue for evilhack, 'gcc -2' issued
warnings about droppables() possibly returning the address of a local
variable (&dummy).  It is mistaken; that never gets returned because
of various checks being performed.  But making 'dummy' be static adds
negligible cost and should shut it up (not verified but no doubt
about viability...).

src/dogmove.c

@@ -23,7 +23,19 @@ static void quickmimic(struct monst *);
 struct obj *
 droppables(struct monst *mon)
 {
-    struct obj *obj, *wep, dummy, *pickaxe, *unihorn, *key;
+    /*
+     * 'key|pickaxe|&c = &dummy' is used to make various creatures
+     * that can't use a key/pick-axe/&c behave as if they are already
+     * holding one so that any other such item in their inventory will
+     * be considered a duplicate and get treated as a normal candidate
+     * for dropping.
+     *
+     * This could be 'auto', but then 'gcc -O2' warns that this function
+     * might return the address of a local variable.  It's mistaken,
+     * &dummy is never returned.  'static' is simplest way to shut it up.
+     */
+    static struct obj dummy;
+    struct obj *obj, *wep, *pickaxe, *unihorn, *key;
 
     dummy = cg.zeroobj;
     dummy.otyp = GOLD_PIECE; /* not STRANGE_OBJECT or tools of interest */