squashfs: do not build on !MMU systems

Fixes:

  http://autobuild.buildroot.org/results/273/2738934fc23219a33fe1e07b3213b4e10266fc79//

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/squashfs/Config.in

@@ -2,6 +2,7 @@ config BR2_PACKAGE_SQUASHFS
 	bool "squashfs"
 	depends on BR2_LARGEFILE
 	depends on BR2_TOOLCHAIN_HAS_THREADS
+	depends on BR2_USE_MMU # fork()
 	select BR2_PACKAGE_SQUASHFS_GZIP if !(BR2_PACKAGE_SQUASHFS_LZMA || BR2_PACKAGE_SQUASHFS_LZO)
 	help
 	  Tools to generate SquashFS filesystems.