linux: turn on -Wextra

sandbox/linux/seccomp/library.cc

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #define XOPEN_SOURCE 500
 #include <algorithm>
 #include <elf.h>
 #include <errno.h>
 #include <errno.h>
 #include <fcntl.h>
@@ skipping 300 matching lines @@
   if (!valid_) {
     return NULL;
   }
   SectionTable::const_iterator iter = section_table_.find(section);
   if (iter == section_table_.end()) {
     return NULL;
   }
   return &iter->second.second;
 }
 
-const int Library::getSectionIndex(const string& section) {
+int Library::getSectionIndex(const string& section) {
   if (!valid_) {
     return -1;
   }
   SectionTable::const_iterator iter = section_table_.find(section);
   if (iter == section_table_.end()) {
     return -1;
   }
   return iter->second.first;
 }
 
@@ skipping 870 matching lines @@
   }
   iter = symbols_.find("__kernel_rt_sigreturn");
   if (iter != symbols_.end() && iter->second.st_value) {
     __kernel_rt_sigreturn = asr_offset_ + iter->second.st_value;
   }
 
   return true;
 }
 
 } // namespace