add stub routines

src/libmemdraw/alloc-stub.c

@@ -0,0 +1,17 @@
+#include <u.h>
+#include <libc.h>
+#include <draw.h>
+#include <memdraw.h>
+
+Memimage*
+allocmemimage(Rectangle r, u32int chan)
+{
+	return _allocmemimage(r, chan);
+}
+
+void
+freememimage(Memimage *m)
+{
+	_freememimage(m);
+}
+