Remove lockdown.h include from lockdown.h

Header guards should prevent recursive inclusion but there is no point in including the file itself.
diff --git a/include/libimobiledevice/lockdown.h b/include/libimobiledevice/lockdown.h
index f22917a..6d390fd 100644
--- a/include/libimobiledevice/lockdown.h
+++ b/include/libimobiledevice/lockdown.h
@@ -31,7 +31,6 @@
 #endif
 
 #include <libimobiledevice/libimobiledevice.h>
-#include <libimobiledevice/lockdown.h>
 
 /** Error Codes */
 typedef enum {