missing.h: __STDC_WANT_LIB_EXT1__

* include/ruby/missing.h: __STDC_WANT_LIB_EXT1__ is necessary for
  memset_s.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52847 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

include/ruby/missing.h

@@ -18,6 +18,8 @@ extern "C" {
 #endif
 #endif
 
+#define __STDC_WANT_LIB_EXT1__ 1
+
 #include "ruby/config.h"
 #include <stddef.h>
 #include <math.h> /* for INFINITY and NAN */