Avoid -Wcast-function-type warnings from casts after GetProcAddress.
Reported by Andy Moreton <andrewjmoreton@gmail.com> in
<https://lists.gnu.org/archive/html/emacs-devel/2018-08/msg00468.html>.
Solution proposed by Eli Zaretskii.
* lib/getaddrinfo.c (GetProcAddress): Cast result to 'void *' first.
* lib/gettimeofday.c (GetProcAddress): Likewise.
* lib/link.c (GetProcAddress): Likewise.
* lib/physmem.c (GetProcAddress): Likewise.
* lib/poll.c (GetProcAddress): Likewise.
* lib/select.c (GetProcAddress): Likewise.
* lib/stat-w32.c (GetProcAddress): Likewise.