zsh: needs mmu

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>

package/zsh/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_ZSH
 	bool "zsh"
+	depends on BR2_USE_MMU # fork()
 	select BR2_PACKAGE_NCURSES
 	help
 	  zsh is a shell designed for interactive use, although it is also