Win32: silenced -Wcast-function-type GCC warning (ticket #1865).

With MinGW-w64, building 64-bit nginx binary with GCC 8 and above
results in warning due to cast of GetProcAddress() result to ngx_wsapoll_pt,
which GCC thinks is incorrect.  Added intermediate cast to "void *" to
silence the warning.

src/os/win32/ngx_win32_init.c

diff --git a/src/os/win32/ngx_win32_init.c b/src/os/win32/ngx_win32_init.c
index 70bee8e..3249fb2 100644
--- a/src/os/win32/ngx_win32_init.c
+++ b/src/os/win32/ngx_win32_init.c
@@ -240,7 +240,7 @@
         goto nopoll;
     }
 
-    WSAPoll = (ngx_wsapoll_pt) GetProcAddress(hmod, "WSAPoll");
+    WSAPoll = (ngx_wsapoll_pt) (void *) GetProcAddress(hmod, "WSAPoll");
     if (WSAPoll == NULL) {
         ngx_log_error(NGX_LOG_NOTICE, log, ngx_errno,
                       "GetProcAddress(\"WSAPoll\") failed");