Make it explicit that aes256gcm_statebytes() returns a rounded value

No actual changes to the returned value

src/libsodium/crypto_aead/aes256gcm/aesni/aead_aes256gcm_aesni.c

@@ -838,7 +838,7 @@ crypto_aead_aes256gcm_abytes(void)
 size_t
 crypto_aead_aes256gcm_statebytes(void)
 {
-    return sizeof(crypto_aead_aes256gcm_state);
+    return (sizeof(crypto_aead_aes256gcm_state) + (size_t) 15U) & ~(size_t) 15U;
 }
 
 #else