Add missing __dead2 to __assert().

__assert() is called when an assertion fails. After printing an error
message, it will call abort(). abort() never returns, hence it has the
__dead2 attribute. Also add this attribute to __assert().

MFC after:	3 weeks

include/assert.h

@@ -58,6 +58,6 @@
 #ifndef _ASSERT_H_
 #define _ASSERT_H_
 __BEGIN_DECLS
-void __assert(const char *, const char *, int, const char *);
+void __assert(const char *, const char *, int, const char *) __dead2;
 __END_DECLS
 #endif /* !_ASSERT_H_ */