separate fbdev-specific code out from minui

Isolate the code that interacts with fbdev, in preparation for adding
a new backend.

Change-Id: I19105e9da1ca6408cebc110f7e2bb5abfb481ee9
diff --git a/minui/minui.h b/minui/minui.h
index 83c121e..b4b57e1 100644
--- a/minui/minui.h
+++ b/minui/minui.h
@@ -17,6 +17,8 @@
 #ifndef _MINUI_H_
 #define _MINUI_H_
 
+#include <sys/types.h>
+
 #include <stdbool.h>
 
 #ifdef __cplusplus