mingw: allow compiling with GCC 8 and DEVELOPER=1

The return type of the `GetProcAddress()` function is `FARPROC` which
evaluates to `long long int (*)()`, i.e. it cannot be cast to the
correct function signature by GCC 8.

To work around that, we first cast to `void *` and go on with our merry
lives.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/poll/poll.c

@@ -149,7 +149,7 @@ win32_compute_revents (HANDLE h, int *p_sought)
     case FILE_TYPE_PIPE:
       if (!once_only)
 	{
-	  NtQueryInformationFile = (PNtQueryInformationFile)
+	  NtQueryInformationFile = (PNtQueryInformationFile)(void (*)(void))
 	    GetProcAddress (GetModuleHandleW (L"ntdll.dll"),
 			    "NtQueryInformationFile");
 	  once_only = TRUE;