Expose offset of private member hidl_string::mBuffer.

Bug: 30575790
Change-Id: Id8ca6cc8a82cc0133732d10aa4fdc107694f7e00
diff --git a/HidlSupport.cpp b/HidlSupport.cpp
index 71ac2b3..27daa94 100644
--- a/HidlSupport.cpp
+++ b/HidlSupport.cpp
@@ -113,6 +113,9 @@
             parentOffset + offsetof(hidl_string, mBuffer));
 }
 
+// static
+const size_t hidl_string::kOffsetOfBuffer = offsetof(hidl_string, mBuffer);
+
 }  // namespace hardware
 }  // namespace android