add missing mode flag needed for Vitis 2023.1 (#1350)

Co-authored-by: Jovan Mitrevski <[email protected]>

Author: JanFSchulte

hls4ml/backends/vitis/vitis_backend.py

@@ -136,7 +136,7 @@ def build(
         with open(tcl_path, 'w') as file:
             file.write(build_opts)
 
-        build_command = 'vitis-run --tcl build_prj.tcl'
+        build_command = 'vitis-run --tcl build_prj.tcl --mode hls'
 
         output_dir = model.config.get_output_dir()
         stdout_log = os.path.join(output_dir, 'build_stdout.log')