gen_defines: close files before exit

In some cases, we've seen the output files be truncated when the
python script has been rebuilt into a .pex before running it --
likely due to buffering.

Closing files explicitly is the right thing to do anyway, so let's
do it.

Signed-off-by: Olof Johansson <olof@lixom.net>

scripts/dts/gen_defines.py

@@ -76,6 +76,8 @@ def main():
 
     print("Devicetree configuration written to " + args.conf_out)
 
+    conf_file.close()
+    header_file.close()
 
 def parse_args():
     # Returns parsed command-line arguments