package/libsigrok: blacklist CS powerpc toolchains

Even though BR2_PACKAGE_LIBSIGROKCXX is selected, the C++
binding is disabled due to lack of C++11 support in the toolchain.

Configure output:
Enabled language bindings:
 - C++............................. no (C++11 compiler required)

So disable libsigrok C++ binding and pulseview for CS powerpc toolchains.

Fixes:
http://autobuild.buildroot.net/results/737/737453011ac0d13ab197258171ffa0f81bc4c368/



[Thomas: use 'depends on !foo && !bar' instead of 'depends on !(foo ||
bar)', seems it looks more logical in this situation.]

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>