minui: Export minui/minui.h.

For libminui static and shared libraries.

Test: build
Change-Id: Ib30dc5e2ef4a3c8b3ca3a0cec68cb65e229a0c16
diff --git a/screen_ui.h b/screen_ui.h
index 3ad6490..b2dcf4a 100644
--- a/screen_ui.h
+++ b/screen_ui.h
@@ -23,7 +23,9 @@
 #include <string>
 
 #include "ui.h"
-#include "minui/minui.h"
+
+// From minui/minui.h.
+struct GRSurface;
 
 // Implementation of RecoveryUI appropriate for devices with a screen
 // (shows an icon + a progress bar, text logging, menu, etc.)