Add RBIMPL_ATTR_NORETURN to unknown_directive

Fixes a compiler warning about the function unknown_directive:

pack.c:160:1: warning: function 'unknown_directive' could be declared with attribute 'noreturn' [-Wmissing-noreturn]

1 files changed, 1 insertions, 0 deletions

diff --git a/pack.c b/pack.c
index f1f474759a..f9b1293c37 100644
--- a/pack.c
+++ b/pack.c
@@ -154,6 +154,7 @@ associated_pointer(VALUE associates, const char *t)
     UNREACHABLE_RETURN(Qnil);
 }
 
+RBIMPL_ATTR_NORETURN()
 static void
 unknown_directive(const char *mode, char type, VALUE fmt)
 {