add lv_theme_zen.h to lv_theme.h

lv_themes/lv_theme.h

@@ -258,6 +258,7 @@ lv_theme_t * lv_theme_get_current(void);
 #include "lv_theme_default.h"
 #include "lv_theme_alien.h"
 #include "lv_theme_night.h"
+#include "lv_theme_zen.h"
 #include "lv_theme_mono.h"
 
 #ifdef __cplusplus