Make method static.

Author: ppisl

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/str/StringBuiltins.java

@@ -1564,7 +1564,7 @@ public String doString(String self, Object width,
             return recursiveNode.executeObject(self, widthCast.execute(width));
         }
 
-        private String zfill(String self, int width) {
+        private static String zfill(String self, int width) {
             int len = self.length();
             if (len >= width) {
                 return self;