Make hmac_ctx optional

Note from future self: actually md_init_ctx will be re-introduced with the
same signature later, and a new function with the additional argument will be
added.
diff --git a/programs/aes/aescrypt2.c b/programs/aes/aescrypt2.c
index e61ba7d..e14acae 100644
--- a/programs/aes/aescrypt2.c
+++ b/programs/aes/aescrypt2.c
@@ -101,7 +101,7 @@
     aes_init( &aes_ctx );
     md_init( &sha_ctx );
 
-    ret = md_init_ctx( &sha_ctx, md_info_from_type( POLARSSL_MD_SHA256 ) );
+    ret = md_init_ctx( &sha_ctx, md_info_from_type( POLARSSL_MD_SHA256 ), 1 );
     if( ret != 0 )
     {
         polarssl_printf( "  ! md_init_ctx() returned -0x%04x\n", -ret );