Update to the new name in usages as well

Somehow the uses of the function were missed when renaming.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/library/cipher.c b/library/cipher.c
index ebb03f4..d0ba01e 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -1145,7 +1145,7 @@
 #if defined(MBEDTLS_CIPHER_PADDING_PKCS7)
         case MBEDTLS_PADDING_PKCS7:
             ctx->add_padding = add_pkcs_padding;
-            ctx->get_padding = get_pkcs_padding;
+            ctx->get_padding = mbedtls_get_pkcs_padding;
             break;
 #endif
 #if defined(MBEDTLS_CIPHER_PADDING_ONE_AND_ZEROS)