Adjust call convention for GetLastError import

Change-Id: Idfd201a8b5f9fe2018b9a3249f5c9444f9b9ef9e

Author: Nikokrock

src/os/win32/gnatcoll-os-win32.ads

@@ -110,7 +110,7 @@ package GNATCOLL.OS.Win32 is
 
    function GetLastError return DWORD
    with Import        => True,
-        Convention    => C,
+        Convention    => Stdcall,
         External_Name => "GetLastError";
 
    Win32_Epoch_Offset : constant LARGE_INTEGER := 11644473600;