message: Add missing NS_ASSUME_NONNULL_{BEGIN,END} macros.
diff: #include "tink/hybrid_encrypt.h"
  +NS_ASSUME_NONNULL_BEGIN
  +
  /**
  * This interface is internal-only. Use TINKHybridEncryptFactory to get an instance that conforms to
  * TINKHybridEncrypt.
  @end
  +NS_ASSUME_NONNULL_END
  +
  #endif // __cplusplus
  