Added missing #define for DS_INCOMPLETE.

diff --git a/include/dsound.h b/include/dsound.h
index 0679901..5c14b56 100644
--- a/include/dsound.h
+++ b/include/dsound.h
@@ -110,6 +110,7 @@
 
 #define DS_OK				0
 #define DS_NO_VIRTUALIZATION            MAKE_HRESULT(0, _FACDS, 10)
+#define DS_INCOMPLETE                   MAKE_HRESULT(0, _FACDS, 20)
 #define DSERR_ALLOCATED			MAKE_DSHRESULT(10)
 #define DSERR_CONTROLUNAVAIL		MAKE_DSHRESULT(30)
 #define DSERR_INVALIDPARAM		E_INVALIDARG