package/mtd: needs zstd

package/mtd: needs zstd

For the target variant, zstd is an optional dependency when ubifs-tools
are enabled. For the host variant, we make it an unconditional
dependency.

Fixes:
http://autobuild.buildroot.org/results/99baf1de106f9c80a32e665263c1e4278097643d (target)
http://autobuild.buildroot.org/results/e3b96704f0b23e82999aa3d6e93233edecbecfe7 (host)

Signed-off-by: Julien BOIBESSOT <julien.boibessot@armadeus.com>
Tested-by: Markus Mayer <mmayer@broadcom.com>
[yann.morin.1998@free.fr: fix the target variant too]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>