Fix MinGW builds by defining HDS_NOSIZING if it's not defined

Blind fix for compilation failures in MinGW builds after the changes of
92a1f643ba which started using HDS_NOSIZING
apparently not present in this compiler SDK headers.

include/wx/msw/missing.h

@@ -198,6 +198,9 @@
 #ifndef HDS_FLAT
     #define HDS_FLAT 0x0200
 #endif
+#ifndef HDS_NOSIZING
+    #define HDS_NOSIZING 0x0800
+#endif
 
 #ifndef HDF_SORTUP
     #define HDF_SORTUP   0x0400