cmd/dist: fix build on openbsd

R=rsc, bradfitz
CC=golang-dev
https://golang.org/cl/5608060

src/cmd/dist/windows.c

@@ -396,10 +396,10 @@ mtime(char *p)
 bool
 isabs(char *p)
 {
-	// c:/ or c:\
+	// "c:/" or "c:\"
 	if(('A' <= p[0] && p[0] <= 'Z') || ('a' <= p[0] && p[0] <= 'z'))
 		return p[1] == ':' && (p[2] == '/' || p[2] == '\\');
-	// / or \
+	// "/" or "\"
 	return p[0] == '/' || p[0] == '\\';
 }