Do not update register.c's timestamp if it is unchanged, otherwise it gets recompiled

every time one of its dependencies is touched.

Change-Id: I8c0c4caea8355748e5abf9b9fe3804834353075f
Reviewed-on: https://code.wireshark.org/review/18304
Reviewed-by: Jörg Mayer <jmayer@loplof.de>

tools/make-dissector-reg.py

@@ -335,7 +335,6 @@ try:
         fh.close()
     else:
         print((final_filename + ' unchanged.'))
-        os.utime(final_filename, None)
 except OSError:
     sys.exit('Unable to write ' + final_filename + '.\n')