Export mbedtls_aes_(en/de)crypt to retain for API compatibility

The commit f5bf7189d303e602992c750c09e429e23c7b2abf made the AES
functions mbedtls_aes_encrypt and mbedtls_aes_decrypt static, changing
the library's API.

This commit reverts this.
diff --git a/library/aes.c b/library/aes.c
index 5e01c4f..5860384 100644
--- a/library/aes.c
+++ b/library/aes.c
@@ -765,6 +765,13 @@
 }
 #endif /* !MBEDTLS_AES_ENCRYPT_ALT */
 
+void mbedtls_aes_encrypt( mbedtls_aes_context *ctx,
+                          const unsigned char input[16],
+                          unsigned char output[16] )
+{
+    mbedtls_internal_aes_encrypt( ctx, input, output );
+}
+
 /*
  * AES-ECB block decryption
  */
@@ -824,6 +831,13 @@
 }
 #endif /* !MBEDTLS_AES_DECRYPT_ALT */
 
+void mbedtls_aes_decrypt( mbedtls_aes_context *ctx,
+                          const unsigned char input[16],
+                          unsigned char output[16] )
+{
+    mbedtls_internal_aes_decrypt( ctx, input, output );
+}
+
 /*
  * AES-ECB block encryption/decryption
  */