zsmalloc: Fix "map_vm_area" undefined reference errors.

This patch adds a MMU dependency to configure the ZSMALLOC in
drivers/staging/zsmalloc/Kconfig. Without this patch, build
system can lead to build failure. This was observed during
randconfig testing, in which ZSMALLOC was enabled w/o MMU being
enabled. Following was the error:

LD      vmlinux
drivers/built-in.o: In function `__zs_map_object':
drivers/staging/zsmalloc/zsmalloc-main.c:650: undefined reference to `map_vm_area'
make: *** [vmlinux] Error 1
Signed-off-by: Manjunath Goudar <csmanjuvijay@gmail.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Seth Jennings <sjenning@linux.vnet.ibm.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: devel@driverdev.osuosl.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zsmalloc/Kconfig

 config ZSMALLOC
 	bool "Memory allocator for compressed pages"
+	depends on MMU
 	default n
 	help
 	  zsmalloc is a slab-based memory allocator designed to store