python-psutil: not available on musl

Fixes:
http://autobuild.buildroot.net/results/365/365c2f0b32ae3cb1d6d4d8f0145500dfadd05c59/
http://autobuild.buildroot.org/results/140/140d0ec9d94f75453c4c82e18803c8d7bffcf6be/



And many more.

The sysinfo structure definition in linux/sysinfo.h (which gets indirectly
included from linux/kernel.h) conflicts with the definition in sys/sysinfo.h
when building against the musl C library, leading to build failures:

arm-linux-gcc -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes \
 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -fPIC -DPSUTIL_VERSION=430 \
  -c psutil/_psutil_linux.c -o build/temp.linux-x86_64-3.5/psutil/_psutil_linux.o
In file included from /home/buildroot/build/instance-0/output/host/usr/arm-buildroot-linux-musleabihf/sysroot/usr/include/linux/kernel.h:4:0,
                 from /home/buildroot/build/instance-0/output/host/usr/arm-buildroot-linux-musleabihf/sysroot/usr/include/linux/ethtool.h:16,
                 from psutil/_psutil_linux.c:35:
/home/buildroot/build/instance-0/output/host/usr/arm-buildroot-linux-musleabihf/sysroot/usr/include/linux/sysinfo.h:7:8: error: redefinition of 'struct sysinfo'
 struct sysinfo {
        ^
In file included from psutil/_psutil_linux.c:21:0:
/home/buildroot/build/instance-0/output/host/usr/arm-buildroot-linux-musleabihf/sysroot/usr/include/sys/sysinfo.h:10:8: note: originally defined here

The suggested solution by the musl developers is to duplicate the needed
structures and defines inline instead of including the kernel headers, which
is unlikely to be acceptable upstream - So instead mark python-psutil (and
its reverse dependencies) as unavailable on musl.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>