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