nasm: Fix compilation with GCC 8

nasm: Fix compilation with GCC 8

host-nasm fails to compile with GCC 8:

In file included from ./include/nasm.h:46,
                 from asm/nasm.c:47:
./include/nasmlib.h:194:1: error: ‘pure’ attribute on function returning ‘void’ [-Werror=attributes]
 void pure_func seg_init(void);
 ^~~~

Pull a patch from upstream that removes the stale declaration of
seg_init, which was eliminated a long time ago.

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>