Include what we use.

Newest libc++ update fixes some of the header cruft.

Change-Id: Ic5019ffcb272274ac3b21627218d65a887074458
diff --git a/runtime/base/allocator.h b/runtime/base/allocator.h
index 8720f0e..2d67c8b 100644
--- a/runtime/base/allocator.h
+++ b/runtime/base/allocator.h
@@ -18,6 +18,7 @@
 #define ART_RUNTIME_BASE_ALLOCATOR_H_
 
 #include <map>
+#include <set>
 
 #include "atomic.h"
 #include "base/macros.h"