dumpon: Move the _Noreturn keyword before the return type

This fixes a warning from GCC 14 when compiling with the native C11
_Noreturn rather than the older GNU C function attribute:

sbin/dumpon/dumpon.c:73:1: error: '_Noreturn' is not at beginning of declaration [-Werror=old-style-declaration]

73 | static void _Noreturn
   | ^~~~~~