Add explicit default copy constructor

Definition of implicit copy constructor is deprecated if it has a
user-declared destructor. This triggers -Wdeprecated compiler warning
and breaks the build in latest Clang update.

Test: Build
Bug: 37752547
Change-Id: I9827e09d0ef846369917192c50db2bb80a0bd0bb
diff --git a/runtime/oat.h b/runtime/oat.h
index a38eebc..6b138ba 100644
--- a/runtime/oat.h
+++ b/runtime/oat.h
@@ -175,6 +175,7 @@
 
   ~OatMethodOffsets();
 
+  OatMethodOffsets(const OatMethodOffsets&) = default;
   OatMethodOffsets& operator=(const OatMethodOffsets&) = default;
 
   uint32_t code_offset_;