Define some additional constants.

diff --git a/include/mmsystem.h b/include/mmsystem.h
index 147c567..4865a13 100644
--- a/include/mmsystem.h
+++ b/include/mmsystem.h
@@ -141,6 +141,9 @@
 
 #define MM_MIM_MOREDATA     0x3CC
 
+#define MM_MIXM_LINE_CHANGE 0x3D0
+#define MM_MIXM_CONTROL_CHANGE 0x3D1
+
 #define MMSYSERR_BASE          0
 #define WAVERR_BASE            32
 #define MIDIERR_BASE           64