gc: various C nits, found by plan 9 compiler.

reported by erik quanstrom.

R=ken2
https://golang.org/cl/181071

src/cmd/gc/go.h

@@ -724,7 +724,7 @@ int	yyparse(void);
 void	addidir(char*);
 void	importfile(Val*, int line);
 void	cannedimports(char*, char*);
-void	unimportfile();
+void	unimportfile(void);
 int32	yylex(void);
 void	typeinit(void);
 void	lexinit(void);
@@ -1174,7 +1174,6 @@ void	cgen_ret(Node *n);
 int	isfat(Type*);
 void	clearfat(Node *n);
 void	cgen(Node*, Node*);
-struct Prog;
 void	gused(Node*);
 void	gdata(Node*, Node*, int);
 void	gdatastring(Node*, Strlit*);

src/cmd/gc/reflect.c

@@ -584,7 +584,7 @@ typename(Type *t)
 	return n;
 }
 
-Sym*
+static Sym*
 dtypesym(Type *t)
 {
 	int ot, n;

src/cmd/gc/subr.c

@@ -2085,7 +2085,7 @@ frame(int context)
 		case ONAME:
 			if(flag)
 				print("--- %s frame ---\n", p);
-			print("%O %S G%ld T\n", n->op, n->sym, n->vargen, n->type);
+			print("%O %S G%ld %T\n", n->op, n->sym, n->vargen, n->type);
 			flag = 0;
 			break;
 

src/cmd/gc/unsafe.c

@@ -30,7 +30,6 @@ unsafenmagic(Node *fn, NodeList *args)
 	}
 	r = args->n;
 
-	n = nod(OLITERAL, N, N);
 	if(strcmp(s->name, "Sizeof") == 0) {
 		typecheck(&r, Erv);
 		tr = r->type;

src/cmd/gc/walk.c

@@ -1072,7 +1072,7 @@ ret:
 	*np = n;
 }
 
-Node*
+static Node*
 makenewvar(Type *t, NodeList **init, Node **nstar)
 {
 	Node *nvar, *nas;