sys/directdraw/gstdirectdrawsink.*: Fix more warnings when compiling with MingW ...

sys/directdraw/gstdirectdrawsink.*: Fix more warnings when compiling with MingW (#439914).

Original commit message from CVS:
Patch by: Vincent Torri  <vtorri at univ-evry fr>
* sys/directdraw/gstdirectdrawsink.c:
(gst_directdraw_sink_buffer_alloc),
(gst_directdraw_sink_show_frame),
(gst_directdraw_sink_check_primary_surface),
(gst_directdraw_sink_check_offscreen_surface),
(EnumModesCallback2), (gst_directdraw_sink_get_ddrawcaps),
(gst_directdraw_sink_surface_create):
* sys/directdraw/gstdirectdrawsink.h:
Fix more warnings when compiling with MingW (#439914).