Add missing return in ScopeGuard::operator=.

This omission did not cause problems earlier due to having not been
instanced.

Change-Id: I7a54f82e06c299902f3bf6b4d3737cc5e30961ad

runtime/hsa-runtime/core/util/utils.h

@@ -158,6 +158,7 @@ class ScopeGuard {
     dismiss_ = rhs.dismiss_;
     release_ = rhs.release_;
     rhs.dismiss_ = true;
+    return *this;
   }
   __forceinline void Dismiss() { dismiss_ = true; }
 

runtime/hsa-runtime/image/util.h

@@ -130,6 +130,7 @@ template <typename lambda> class ScopeGuard {
     dismiss_ = rhs.dismiss_;
     release_ = rhs.release_;
     rhs.dismiss_ = true;
+    return *this;
   }
   __forceinline void Dismiss() { dismiss_ = true; }