[filesapp] SWA main.html autogen: import print_function from future

Followup to CL:2585261. Use idiomatic import of print function: import
it from the __future__.

Bug: 1113981
Change-Id: Ib5f7b87c193c9a9de60d25f9e82a4a6e0ff55bc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593848Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837393}

chromeos/components/file_manager/resources/gen_main_html.py

@@ -6,6 +6,8 @@
 
 """Generate SWA files app main.html from files app main.html"""
 
+from __future__ import print_function
+
 import fileinput
 import optparse
 import os