maint: don't use the deprecated stdnoreturn module

* bootstrap.conf (gnulib_modules): Remove stdnoreturn.
* src/hello.c: Don't include <stdnoreturn.h>. Gnulib will define
_Noreturn in <config.h> if the compiler does not support the C11
keyword.

diff --git a/bootstrap.conf b/bootstrap.conf
line changes: +0/-1
index 2f41186..2f2470e
--- a/bootstrap.conf
+++ b/bootstrap.conf

@@ -36,7 +36,6 @@ gnulib_modules="
     progname
     propername
     readme-release
-    stdnoreturn
     update-copyright
     version-etc-fsf
     wchar

diff --git a/src/hello.c b/src/hello.c
line changes: +0/-1
index e51316d..35151df
--- a/src/hello.c
+++ b/src/hello.c

@@ -20,7 +20,6 @@
 #include <errno.h>
 #include <error.h>
 #include <getopt.h>
-#include <stdnoreturn.h>
 #include <wchar.h>
 
 #include "system.h"