package/upmpdcli: disable for PowerPC CS toolchains

package/upmpdcli: disable for PowerPC CS toolchains

Those toolchains use gcc <= 4.5, which is too old as they are missing to
construct an empty std::shared_ptr with a nullptr (0) as an argument. This
is only introduced with gcc-4.6.

Fixes:
http://autobuild.buildroot.net/results/faf/faf1b071c0a5bd8395d79abf2e6a32169ad7f136
http://autobuild.buildroot.net/results/247/247b3f1ac3d3a9712e9410eef298fce8d885f92c
http://autobuild.buildroot.net/results/de6/de601c4d4dd7922895892d78d3a37902fb98c7be
http://autobuild.buildroot.net/results/a4d/a4d7bf9863c134f59d285616b9c1f78201bd1a3e

and many more.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>