package/upmpdcli: needs MMU

package/upmpdcli: needs MMU

... because it uses fork().

Fixes:
    http://autobuild.buildroot.org/results/9ae/9ae5c773e62bbe84f2d0495f074d659fa3dcd55b/
    http://autobuild.buildroot.org/results/8af/8afb9f96bb827b723b859ff73cdf674ca6434e7c/
    http://autobuild.buildroot.org/results/abe/abe9be3b78f26f56035ec805a86ab33b1d145a02/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Jörg Krause <joerg.krause@embedded.rocks>
[Thomas: propagate to the Config.in comment.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>