AvoidBindCheck.cpp: Fix GCC 5.3 build errors

It was failing with:

clang-tools-extra/clang-tidy/modernize/AvoidBindCheck.cpp:61:29:
error: declaration of ‘clang::tidy::modernize::{anonymous}::CaptureMode clang::tidy::modernize::{anonymous}::BindArgument::CaptureMode’ [-fpermissive]
   CaptureMode CaptureMode = CM_None;
                             ^
clang-tools-extra/clang-tidy/modernize/AvoidBindCheck.cpp:38:6:
error: changes meaning of ‘CaptureMode’ from ‘enum clang::tidy::modernize::{anonymous}::CaptureMode’ [-fpermissive]
 enum CaptureMode { CM_None, CM_ByRef, CM_ByValue, CM_InitExpression };
      ^