refactor: remove `stdnoreturn.h` includes

The `noreturn` macro and `_Noreturn` keywords are deprecated in C23. The
reccommended way to indicate noreturn is now `[[noreturn]]`, which uses
the new C23 attribute syntax.

Change-Id: Ib923fce8bbe97597aa5ed27bb0962a107191022b
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/panic.c b/src/panic.c
index cfd9a37..a810916 100644
--- a/src/panic.c
+++ b/src/panic.c
@@ -18,7 +18,7 @@
  *
  * TODO: Determine if we want to omit strings on non-debug builds.
  */
-noreturn void panic(const char *fmt, ...)
+[[noreturn]] void panic(const char *fmt, ...)
 {
 	struct va_list_wrapper args;