Pacify g++ about a set-but-unused variable.

This fixes the MIPS32 build.

Change-Id: Ic1e6d4627d69a3701c2f3e8fdd784d97a4e41ba5
diff --git a/runtime/thread.cc b/runtime/thread.cc
index 2176444..2a7cd07 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -561,6 +561,7 @@
 
   // Read every page from the high address to the low.
   volatile uint8_t dont_optimize_this;
+  UNUSED(dont_optimize_this);
   for (uint8_t* p = stack_top; p >= pregion; p -= kPageSize) {
     dont_optimize_this = *p;
   }