Fix mips64 build: Add missing "=".

Change-Id: I457732945ae8f554ed49943c913b8c4fcc2a4d18
diff --git a/runtime/arch/mips64/context_mips64.cc b/runtime/arch/mips64/context_mips64.cc
index 597a74c..d808c9e 100644
--- a/runtime/arch/mips64/context_mips64.cc
+++ b/runtime/arch/mips64/context_mips64.cc
@@ -38,7 +38,7 @@
 }
 
 void Mips64Context::FillCalleeSaves(const StackVisitor& fr) {
-  ArtCode code fr.GetCurrentCode();
+  ArtCode code = fr.GetCurrentCode();
   const QuickMethodFrameInfo frame_info = code.GetQuickFrameInfo();
   int spill_pos = 0;