libunwindstack: expose UnwindStackMap::GetFunctionName.

Test: mma
Change-Id: I8736ff5c2ed7c5e64eb68df5f4eccfed614612c7
diff --git a/libbacktrace/UnwindStackMap.h b/libbacktrace/UnwindStackMap.h
index b93b340..2a8af83 100644
--- a/libbacktrace/UnwindStackMap.h
+++ b/libbacktrace/UnwindStackMap.h
@@ -34,6 +34,8 @@
 
   void FillIn(uintptr_t addr, backtrace_map_t* map) override;
 
+  virtual std::string GetFunctionName(uintptr_t pc, uintptr_t* offset) override;
+
   unwindstack::Maps* stack_maps() { return stack_maps_.get(); }
 
   const std::shared_ptr<unwindstack::Memory>& process_memory() { return process_memory_; }