Fix snprintf usage again

Sigh, switching between branches is error-prone
diff --git a/programs/pkey/rsa_sign.c b/programs/pkey/rsa_sign.c
index 7bd0fad..c9bdd3a 100644
--- a/programs/pkey/rsa_sign.c
+++ b/programs/pkey/rsa_sign.c
@@ -141,7 +141,7 @@
     /*
      * Write the signature into <filename>.sig
      */
-    snprintf( filename, sizeof( filename ), "%s.sig", argv[1] );
+    polarssl_snprintf( filename, sizeof( filename ), "%s.sig", argv[1] );
 
     if( ( f = fopen( filename, "wb+" ) ) == NULL )
     {