mmsd: Incorrect gsize to int conversion

cc @evangelos.tzaras

In commit 9c627f52

max_attach_size was changed from an int to a gsize. However, max_attach_size is taken from chatty here:

https://source.puri.sm/Librem5/chatty/-/blob/master/src/mm/chatty-mmsd.c#L1367

if (g_variant_dict_lookup (&dict, "TotalMaxAttachmentSize", "i", &max_attach_total_size))

And the conversation from int to gsize is incorrect. If mmsd-tng is using 11000000, chatty reports:

12:47:50.7616          chatty-mmsd[12339]:   DEBUG: TotalMaxAttachmentSize is set to 187647122262240

This is undesirable, as now Chatty will try to send images to mmsd-tng that are too large.