Cast size_t to to gulong as the length argument to g_snprintf().

Sadly, the GTK+ folks decided not to use size_t for the size argument,
so it doesn't do the right thing on LLP64 platforms such as Windows.

Change-Id: I2aa9096215c488b48f1cf68d2a285a48abb6f07f
Reviewed-on: https://code.wireshark.org/review/4584
Reviewed-by: Guy Harris <guy@alum.mit.edu>