add stub routines

src/libmemdraw/unload-stub.c

@@ -0,0 +1,10 @@
+#include <u.h>
+#include <libc.h>
+#include <draw.h>
+#include <memdraw.h>
+
+int
+unloadmemimage(Memimage *i, Rectangle r, uchar *data, int ndata)
+{
+	return _unloadmemimage(i, r, data, ndata);
+}