UISimulator - prevent multiple definition of usb_inserted when USB_NONE is defined

This fixes simulator build errors on mini2440


git-svn-id: svn://svn.rockbox.org/rockbox/trunk@23723 a1c6a512-1295-4272-9138-f99709370657
diff --git a/uisimulator/common/stubs.c b/uisimulator/common/stubs.c
index 2218c49..d1ec4ec 100644
--- a/uisimulator/common/stubs.c
+++ b/uisimulator/common/stubs.c
@@ -250,10 +250,12 @@
 }
 #endif /* CONFIG_CHARGING */
 
+#ifndef USB_NONE
 bool usb_inserted(void)
 {
     return false;
 }
+#endif
 
 #ifdef HAVE_REMOTE_LCD_TICKING
 void lcd_remote_emireduce(bool state)