Limit exposure of skin engine internals

Drop wps_internals.h from skin_engine.h. The WPS and to a lesser
extent the radio screen are too tightly integrated to drop their
dependency on wps_internals.h, unfortunately. Skinned lists, for
obvious reasons, also need access to the internals.

Change-Id: I00a55aa423900f9ad22edccbe2fc1910af380e38

apps/radio/radio.c

@@ -56,6 +56,7 @@
 #include "statusbar-skinned.h"
 #include "playback.h"
 #include "presets.h"
+#include "skin_engine/wps_internals.h"
 
 #if CONFIG_TUNER
 

apps/radio/radio_skin.c

@@ -36,6 +36,7 @@
 #include "sound.h"
 #include "misc.h"
 #endif
+#include "skin_engine/wps_internals.h"
 
 
 char* default_radio_skin(enum screen_type screen)