Fix missing header inclusion in art/runtime/thread.cc.

The file art/runtime/thread.cc was missing an #include of
art/runtime/gc/accounting/heap_bitmap-inl.h, which is
required by art::ReferenceMapVisitor::VisitDeclaringClass to
find the (inlined) implementation of
art::gc::accounting::HeapBitmap::Test.

This fixes the MIPS32 build.

Change-Id: I00a8bc85d8b32cfed02309e13e437f0cfd0d493e
diff --git a/runtime/thread.cc b/runtime/thread.cc
index 4dc6d57..42b5a4a 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -44,6 +44,7 @@
 #include "entrypoints/quick/quick_alloc_entrypoints.h"
 #include "gc_map.h"
 #include "gc/accounting/card_table-inl.h"
+#include "gc/accounting/heap_bitmap-inl.h"
 #include "gc/allocator/rosalloc.h"
 #include "gc/heap.h"
 #include "gc/space/space-inl.h"