Properly define true/false when defining __bool_true_false_are_defined

for filters which pull in mfapi.h before stdbool.h.  Issue reported by
Petr Rehor, maintainer of amavisd-milter port.

MFC after:	3 days

contrib/sendmail/include/libmilter/mfapi.h

@@ -96,6 +96,8 @@ typedef int	sfsistat;
 #  ifndef bool
 #   ifndef __bool_true_false_are_defined
 typedef int	bool;
+#    define false	0
+#    define true	1
 #    define __bool_true_false_are_defined	1
 #   endif /* ! __bool_true_false_are_defined */
 #  endif /* bool */