Fix the compilation of gdbjit

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/18023005

Patch from Haitao Feng <haitao.feng@intel.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15351 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/gdb-jit.cc

@@ -2084,7 +2084,6 @@ void GDBJITInterface::AddCode(const char* name,
 
   AddUnwindInfo(&code_desc);
   Isolate* isolate = code->GetIsolate();
-  Zone* zone = code->GetIsolate()->runtime_zone();
   JITCodeEntry* entry = CreateELFObject(&code_desc, isolate);
   ASSERT(!IsLineInfoTagged(entry));