stdbool.h should define __bool_true_false_are_defined even for C++

while the incorporation of this requirement from C99 into C++11 was
likely an accident, some software expects it to be defined, and it
doesn't hurt. if the requirement is removed, then presumably
__bool_true_false_are_defined would just be in the implementation
namespace and thus defining it would still be legal.

include/stdbool.h

@@ -7,8 +7,8 @@
 #define false 0
 #define bool _Bool
 
-#define __bool_true_false_are_defined 1
-
 #endif
 
+#define __bool_true_false_are_defined 1
+
 #endif