ndisc6: needs mmu

Fix below build error if !BR2_USE_MMU:

tcpspray.o: In function `tcpspray':
tcpspray.c:(.text+0x4a8): undefined reference to `fork'
collect2: ld returned 1 exit status
make[3]: *** [tcpspray] Error 1

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>