Make the wrapped Value of Status const accessible

Test: compiles
Change-Id: Ie5ceadd64a1b24edbc628ca421fcbf91a78272cf
diff --git a/include/hidl/Status.h b/include/hidl/Status.h
index 59e4d65..6dde65c 100644
--- a/include/hidl/Status.h
+++ b/include/hidl/Status.h
@@ -153,7 +153,7 @@
 public:
       Return(T v) : val{v} {}
       Return(Status s) : status(s) {}
-      operator T() { return val; }
+      operator T() const { return val; }
       const Status& getStatus() const {
           return status;
       }