[Pidgin] #16740: MSVC fixes for libpurple 2.10.11

[Pidgin]

#16740: MSVC fixes for libpurple 2.10.11
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 Reporter:  clokep   |      Owner:  QuLogic
     Type:  defect   |     Status:  new
Milestone:           |  Component:  MSN
  Version:  2.10.11  |   Keywords:
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 While upgrading Instantbird to libpurple 2.10.11 I ran into some code MSVC
 wouldn't compile. It was all fairly minor changes (and actually seems to
 be changes that Instantbird had had forever before I stomped on them).

 The attached patch includes changes for MSN. There were also issues with
 compiling libgadu, but I'll do a separate PR to upstream for that (unless
 you'd like me to include them here too).

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Ticket URL: <https://developer.pidgin.im/ticket/16740>
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