Include stdbool.h also from prefs.h if included from C file

Currently not mandatory in our code because we never include
prefs.h from a C file today but for the future this could avoid confusion.

Signed-off-by: Stefan Fuchs <sfuchs@gmx.de>

core/pref.h

@@ -4,6 +4,8 @@
 
 #ifdef __cplusplus
 extern "C" {
+#else
+#include <stdbool.h>
 #endif
 
 #include "units.h"