dmusic: Implement IDirectMusicBufferImpl_Flush.

dlls/dmusic/buffer.c

diff --git a/dlls/dmusic/buffer.c b/dlls/dmusic/buffer.c
index ca53db3..bf60470 100644
--- a/dlls/dmusic/buffer.c
+++ b/dlls/dmusic/buffer.c
@@ -82,7 +82,9 @@
 {
     IDirectMusicBufferImpl *This = impl_from_IDirectMusicBuffer(iface);
 
-    FIXME("(%p): stub\n", This);
+    TRACE("(%p)->()\n", iface);
+
+    This->write_pos = 0;
 
     return S_OK;
 }