app-emulation/spice: Added dev-python/six build dependency

app-emulation/spice: Added dev-python/six build dependency

Spice requires Six, the Python 2 and 3 compatibility library, otherwise
the build will fail with the following error:

[...]
make[4]: Entering directory '/var/tmp/portage/app-emulation/spice-0.12.6/work/spice-0.12.6/spice-common/common'
/usr/bin/python2.7 /usr/lib64/spice-protocol/spice_codegen.py --generate-demarshallers --client --include common/messages.h /usr/lib64/spice-protocol/spice.proto generated_client_demarshallers.c >/dev/null
Traceback (most recent call last):
  File "/usr/lib64/spice-protocol/spice_codegen.py", line 7, in <module>
    from python_modules import spice_parser
  File "/usr/lib64/spice-protocol/python_modules/spice_parser.py", line 1, in <module>
    import six
ImportError: No module named six
Makefile:794: recipe for target 'generated_client_demarshallers.c' failed
make[4]: *** [generated_client_demarshallers.c] Error 1
make[4]: Leaving directory '/var/tmp/portage/app-emulation/spice-0.12.6/work/spice-0.12.6/spice-common/common'
Makefile:445: recipe for target 'all-recursive' failed
make[3]: *** [all-recursive] Error 1