Fix below build error if !BR2_USE_MMU:
exec.o: In function `zfork':
exec.c:(.text+0x150c): undefined reference to `fork'
collect2: ld returned 1 exit status
make[2]: *** [zsh] Error 1
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
