Cygwin: gencat: define __dead

NetBSD defines __dead as __attribute__((__noreturn__)).  Add a matching
macro expression.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
(cherry picked from commit ede7cbc)

Author: github-cygwin

winsup/utils/gencat.c

@@ -96,6 +96,10 @@ up-to-date.  Many thanks.
 #define NL_MSGMAX 2048
 #endif
 
+#ifndef __dead
+#define __dead __attribute__((__noreturn__))
+#endif
+
 struct _msgT {
 	long    msgId;
 	char   *str;