milkymist: fix tmu2.c build failure (missing error.h include)

Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Acked-by: NMichael Walle <michael@walle.cc>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/display/milkymist-tmu2.c

@@ -29,6 +29,7 @@
 #include "hw/sysbus.h"
 #include "trace.h"
 #include "qemu/error-report.h"
+#include "qapi/error.h"
 
 #include <X11/Xlib.h>
 #include <epoxy/gl.h>