Add missing MBEDTLS_DEPRECATED_REMOVED guards

Add missing MBEDTLS_DEPRECATED_REMOVED guards around the definitions
of mbedtls_aes_decrypt and mbedtls_aes_encrypt.
This fixes the build under -Wmissing-prototypes -Werror.

Fixes #1388
diff --git a/library/aes.c b/library/aes.c
index dba4a5f..3d2eac8 100644
--- a/library/aes.c
+++ b/library/aes.c
@@ -765,12 +765,14 @@
 }
 #endif /* !MBEDTLS_AES_ENCRYPT_ALT */
 
+#if !defined(MBEDTLS_DEPRECATED_REMOVED)
 void mbedtls_aes_encrypt( mbedtls_aes_context *ctx,
                           const unsigned char input[16],
                           unsigned char output[16] )
 {
     mbedtls_internal_aes_encrypt( ctx, input, output );
 }
+#endif /* !MBEDTLS_DEPRECATED_REMOVED */
 
 /*
  * AES-ECB block decryption
@@ -831,12 +833,14 @@
 }
 #endif /* !MBEDTLS_AES_DECRYPT_ALT */
 
+#if !defined(MBEDTLS_DEPRECATED_REMOVED)
 void mbedtls_aes_decrypt( mbedtls_aes_context *ctx,
                           const unsigned char input[16],
                           unsigned char output[16] )
 {
     mbedtls_internal_aes_decrypt( ctx, input, output );
 }
+#endif /* !MBEDTLS_DEPRECATED_REMOVED */
 
 /*
  * AES-ECB block encryption/decryption