pc-bios/s390-ccw: Silence warning from Clang by marking panic() as noreturn

pc-bios/s390-ccw: Silence warning from Clang by marking panic() as noreturn

When compiling the s390-ccw bios with Clang, the compiler emits a warning:

 pc-bios/s390-ccw/main.c:210:5: warning: variable 'found' is used uninitialized
  whenever switch default is taken [-Wsometimes-uninitialized]
     default:
     ^~~~~~~
 pc-bios/s390-ccw/main.c:214:16: note: uninitialized use occurs here
     IPL_assert(found, "Boot device not found\n");
                ^~~~~

It's a false positive, it only happens because Clang is not smart enough
to see that the panic() function in the "default:" case can never return.

Anyway, let's explicitely mark panic() with "noreturn" to shut up the
warning.

Message-Id: <20210502174836.838816-2-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>